O-Ring Size Chart 2026 — AS568 Dash Sizes, Metric ISO 3601 & Material Guide

Q: What does the AS568 dash number mean?

The dash number is a three-digit code from the AS568 standard. The first digit indicates the cross-section series (0=1.78mm, 1=2.62mm, 2=3.53mm, 3=5.33mm, 4=6.99mm). The remaining digits indicate the inside diameter step. For example, -214 is the 200-series (CS=3.53mm) with ID=24.99mm (0.984in).

Q: What is the most common O-ring size?

In AS568 standard, the -200 series (CS = 3.53mm) is most widely used. For metric O-rings, 20x2.0mm and 20x3.0mm are among the most frequently specified in European and Asian equipment.

Q: Can I substitute a metric O-ring for an AS568 size?

Only if the dimensions give correct groove compression. Some are near-equivalent (1.78mm vs 1.8mm CS), but always verify groove fill. Never assume interchangeability for dynamic seals, aerospace, or high-pressure applications.

By myringsizecalculator.com · Updated 2026 · 12 min read

⚡ Quick Answer: O-Ring Sizing

Every O-ring is defined by three dimensions: ID (Inside Diameter), CS (Cross-Section thickness), and OD (Outside Diameter). The formula is always OD = ID + 2 × CS. In the USA the AS568 standard assigns each size a dash number (e.g. -214). In Europe and Asia the ISO 3601 metric standard uses ID × CS in mm (e.g. 25 × 3.0 mm). When between sizes, always choose the size that gives a 10–15% compression of the cross-section in the groove.

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All 369 AS568 sizes · ISO 3601 metric · Material guide · No sign-up required

1. Understanding O-Ring Dimensions

Before reading any O-ring chart, you need to understand the three dimensions that define every O-ring:

ID (Inside Diameter) — the diameter of the hole in the centre of the O-ring. This is the primary dimension used to specify an O-ring for a shaft or rod seal.
CS (Cross-Section) — the diameter of the circular cord that makes up the ring itself. Sometimes called the "width" or "thickness." This controls how much the ring compresses in the groove.
OD (Outside Diameter) — the total outer diameter. Always calculated, never directly specified: OD = ID + 2 × CS.

📌 The Golden Formula

OD = ID + 2 × CS

Example: An O-ring with ID = 25.07 mm and CS = 3.53 mm has OD = 25.07 + (2 × 3.53) = 32.13 mm.
Know any two of the three dimensions and you can always calculate the third.

2. AS568 Standard O-Ring Size Chart

AS568 (published by SAE International) is the dominant O-ring standard in North America and for US military/aerospace applications. It organises O-rings into 6 series by cross-section size, with a three-digit dash number identifying each size. The first digit indicates the series; the remaining digits indicate the ID step within that series.

-0xx

CS = 1.78 mm (0.070″)

Dash 001–049 · 49 sizes

-1xx

CS = 2.62 mm (0.103″)

Dash 102–178 · 46 sizes

-2xx

CS = 3.53 mm (0.139″)

Dash 201–284 · 84 sizes

-3xx

CS = 5.33 mm (0.210″)

Dash 309–395 · 87 sizes

-4xx

CS = 6.99 mm (0.275″)

Dash 425–475 · 51 sizes

-9xx

Boss seals (varied CS)

SAE straight-thread ports

AS568 -0xx Series (CS = 1.78 mm / 0.070″) — Most Common Small Sizes

Dash No.	ID (in)	ID (mm)	CS (in)	CS (mm)	OD (in)	OD (mm)
-001	0.029	0.74	0.040	1.02	0.109	2.77
-002	0.042	1.07	0.050	1.27	0.142	3.61
-003	0.056	1.42	0.060	1.52	0.176	4.47
-004	0.070	1.78	0.070	1.78	0.210	5.33
-005	0.101	2.57	0.070	1.78	0.241	6.12
-006	0.114	2.90	0.070	1.78	0.254	6.45
-007	0.145	3.68	0.070	1.78	0.285	7.24
-008	0.176	4.47	0.070	1.78	0.316	8.03
-009	0.208	5.28	0.070	1.78	0.348	8.84
-010	0.239	6.07	0.070	1.78	0.379	9.63
-011	0.301	7.65	0.070	1.78	0.441	11.20
-012	0.364	9.25	0.070	1.78	0.504	12.80
-013	0.426	10.82	0.070	1.78	0.566	14.38
-014	0.489	12.42	0.070	1.78	0.629	15.98
-015	0.551	13.99	0.070	1.78	0.691	17.55
-016	0.614	15.60	0.070	1.78	0.754	19.15
-017	0.676	17.17	0.070	1.78	0.816	20.73
-018	0.739	18.77	0.070	1.78	0.879	22.33
-019	0.801	20.35	0.070	1.78	0.941	23.91
-020	0.864	21.95	0.070	1.78	1.004	25.50

AS568 -1xx Series (CS = 2.62 mm / 0.103″)

Dash No.	ID (in)	ID (mm)	CS (in)	CS (mm)	OD (in)	OD (mm)
-102	0.049	1.24	0.103	2.62	0.255	6.48
-103	0.081	2.06	0.103	2.62	0.287	7.29
-104	0.112	2.84	0.103	2.62	0.318	8.08
-106	0.174	4.42	0.103	2.62	0.380	9.65
-108	0.237	6.02	0.103	2.62	0.443	11.25
-109	0.299	7.59	0.103	2.62	0.505	12.83
-110	0.362	9.19	0.103	2.62	0.568	14.43
-111	0.424	10.77	0.103	2.62	0.630	16.00
-112	0.487	12.37	0.103	2.62	0.693	17.60
-113	0.549	13.94	0.103	2.62	0.755	19.18
-114	0.612	15.54	0.103	2.62	0.818	20.78
-116	0.737	18.72	0.103	2.62	0.943	23.95
-118	0.862	21.89	0.103	2.62	1.068	27.13
-120	0.987	25.07	0.103	2.62	1.193	30.30
-125	1.237	31.42	0.103	2.62	1.443	36.65
-130	1.487	37.77	0.103	2.62	1.693	43.00
-150	2.487	63.17	0.103	2.62	2.693	68.40

AS568 -2xx Series (CS = 3.53 mm / 0.139″) — Most Widely Used

Dash No.	ID (in)	ID (mm)	CS (in)	CS (mm)	OD (in)	OD (mm)
-201	0.171	4.34	0.139	3.53	0.449	11.40
-202	0.234	5.94	0.139	3.53	0.512	13.00
-204	0.359	9.12	0.139	3.53	0.637	16.18
-206	0.484	12.29	0.139	3.53	0.762	19.35
-208	0.609	15.47	0.139	3.53	0.887	22.53
-210	0.734	18.64	0.139	3.53	1.012	25.70
-212	0.859	21.82	0.139	3.53	1.137	28.88
-214	0.984	24.99	0.139	3.53	1.262	32.05
-216	1.109	28.17	0.139	3.53	1.387	35.23
-218	1.234	31.34	0.139	3.53	1.512	38.40
-220	1.359	34.52	0.139	3.53	1.637	41.58
-222	1.484	37.69	0.139	3.53	1.762	44.75
-224	1.609	40.87	0.139	3.53	1.887	47.93
-226	1.734	44.04	0.139	3.53	2.012	51.10
-228	1.859	47.22	0.139	3.53	2.137	54.28
-230	1.984	50.39	0.139	3.53	2.262	57.45
-232	2.109	53.57	0.139	3.53	2.387	60.63
-234	2.234	56.74	0.139	3.53	2.512	63.80
-236	2.359	59.92	0.139	3.53	2.637	66.98
-238	2.484	63.09	0.139	3.53	2.762	70.15
-240	2.609	66.27	0.139	3.53	2.887	73.33
-244	2.859	72.62	0.139	3.53	3.137	79.68
-248	3.109	78.97	0.139	3.53	3.387	86.03
-260	3.859	98.02	0.139	3.53	4.137	105.08
-284	8.984	228.19	0.139	3.53	9.262	235.25

AS568 -3xx Series (CS = 5.33 mm / 0.210″) — Selected Sizes

Dash No.	ID (in)	ID (mm)	CS (in)	CS (mm)	OD (in)	OD (mm)
-309	0.551	13.99	0.210	5.33	0.971	24.66
-310	0.614	15.60	0.210	5.33	1.034	26.26
-312	0.739	18.77	0.210	5.33	1.159	29.43
-314	0.864	21.95	0.210	5.33	1.284	32.61
-316	0.989	25.12	0.210	5.33	1.409	35.79
-318	1.114	28.30	0.210	5.33	1.534	38.96
-320	1.239	31.47	0.210	5.33	1.659	42.14
-325	1.489	37.82	0.210	5.33	1.909	48.49
-330	1.739	44.17	0.210	5.33	2.159	54.84
-340	2.239	56.87	0.210	5.33	2.659	67.54
-350	2.739	69.57	0.210	5.33	3.159	80.24
-360	3.239	82.27	0.210	5.33	3.659	92.94
-375	3.989	101.32	0.210	5.33	4.409	111.99
-395	14.984	380.59	0.210	5.33	15.404	391.26

AS568 -4xx Series (CS = 6.99 mm / 0.275″) — Selected Sizes

Dash No.	ID (in)	ID (mm)	CS (in)	CS (mm)	OD (in)	OD (mm)
-425	1.239	31.47	0.275	6.99	1.789	45.44
-430	1.489	37.82	0.275	6.99	2.039	51.79
-432	1.614	40.99	0.275	6.99	2.164	54.96
-436	1.864	47.35	0.275	6.99	2.414	61.32
-440	2.114	53.70	0.275	6.99	2.664	67.67
-444	2.364	60.05	0.275	6.99	2.914	74.02
-450	2.739	69.57	0.275	6.99	3.289	83.54
-460	3.239	82.27	0.275	6.99	3.789	96.24
-475	25.940	659.00	0.275	6.99	26.490	672.99

3. ISO 3601 Metric O-Ring Size Chart

Outside North America, O-rings are specified using the ISO 3601 metric standard. Metric O-rings are written as ID × CS in millimetres — for example, 20 × 2.0 means 20 mm inside diameter, 2.0 mm cross-section, 24 mm OD. The most common cross-sections are 1.0, 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, and 5.0 mm.

⚠️ Metric and AS568 Sizes Are NOT Interchangeable Some cross-section thicknesses are close but not identical — 1.78 mm (AS568 -0xx) vs 1.8 mm (metric), 2.62 mm vs 2.65 mm, 3.53 mm vs 3.55 mm. These small differences can cause incorrect groove compression or leakage. Always match the O-ring to the standard the groove was designed for.

Metric O-Rings — Common 2.0 mm Cross-Section Sizes

Designation	ID (mm)	CS (mm)	OD (mm)	ID (in)	CS (in)	Typical Application
3 × 2.0	3.00	2.00	7.00	0.118	0.079	Miniature valves
4 × 2.0	4.00	2.00	8.00	0.157	0.079	Fittings, sensors
5 × 2.0	5.00	2.00	9.00	0.197	0.079	Small bore pneumatics
6 × 2.0	6.00	2.00	10.00	0.236	0.079	Hydraulic fittings
8 × 2.0	8.00	2.00	12.00	0.315	0.079	Rod seals, pistons
10 × 2.0	10.00	2.00	14.00	0.394	0.079	Very widely used
12 × 2.0	12.00	2.00	16.00	0.472	0.079	Very widely used
14 × 2.0	14.00	2.00	18.00	0.551	0.079	Pump housings
15 × 2.0	15.00	2.00	19.00	0.591	0.079	Cylinder bores
16 × 2.0	16.00	2.00	20.00	0.630	0.079	Valves, pneumatics
18 × 2.0	18.00	2.00	22.00	0.709	0.079	Hydraulic cylinders
20 × 2.0	20.00	2.00	24.00	0.787	0.079	Most common overall
22 × 2.0	22.00	2.00	26.00	0.866	0.079	General industrial
25 × 2.0	25.00	2.00	29.00	0.984	0.079	Fittings, flanges
30 × 2.0	30.00	2.00	34.00	1.181	0.079	Larger bore seals
35 × 2.0	35.00	2.00	39.00	1.378	0.079	Flanged connections
40 × 2.0	40.00	2.00	44.00	1.575	0.079	Pipe fittings
50 × 2.0	50.00	2.00	54.00	1.969	0.079	Medium bore hydraulics

Metric O-Rings — Common 3.0 mm Cross-Section Sizes

Designation	ID (mm)	CS (mm)	OD (mm)	ID (in)	Typical Use
8 × 3.0	8.00	3.00	14.00	0.315	Hydraulic cylinders
10 × 3.0	10.00	3.00	16.00	0.394	Fittings & connectors
12 × 3.0	12.00	3.00	18.00	0.472	Pumps & valves
15 × 3.0	15.00	3.00	21.00	0.591	Very widely used
20 × 3.0	20.00	3.00	26.00	0.787	Very widely used
25 × 3.0	25.00	3.00	31.00	0.984	Industrial general
30 × 3.0	30.00	3.00	36.00	1.181	Cylinder bores
40 × 3.0	40.00	3.00	46.00	1.575	Hydraulic systems
50 × 3.0	50.00	3.00	56.00	1.969	Flanges
60 × 3.0	60.00	3.00	66.00	2.362	Large bore pneumatics
80 × 3.0	80.00	3.00	86.00	3.150	Pipe seals
100 × 3.0	100.00	3.00	106.00	3.937	Large flanged joints

4. How to Measure an O-Ring

If you have an existing O-ring you need to match, measure its ID and CS — never the OD, which is a derived dimension and varies with stretch.

Measure the Inside Diameter (ID): Lay the O-ring flat on a ruler. Measure straight across the inside of the ring from inner edge to inner edge. For small or irregular O-rings, use digital calipers for accuracy.
Measure the Cross-Section (CS): Using calipers, measure the diameter of the cord (the rubber part) at any point around the ring. This is the CS. Do not measure a stretched or compressed ring.
Calculate the OD: OD = ID + 2 × CS. Use this to verify your measurement against the part drawing or groove dimensions.
Match to a standard size: Search the AS568 or ISO 3601 tables above for the closest matching ID and CS combination. If your measurement falls between two sizes, check the groove dimensions to determine which size gives 10–15% cross-section compression.
Verify with the dash number: For AS568, confirm the dash number against the part drawing or OEM specification. Never substitute a metric O-ring into an inch groove or vice versa without verifying groove fill.

✅ Measuring Tip: Use Digital Calipers O-ring dimensions are typically specified to three decimal places in inches (e.g. 0.984 in) or one decimal in mm (25.0 mm). A standard ruler cannot give you this accuracy. Digital calipers are inexpensive (under $20) and essential for matching O-ring sizes accurately. Measure the CS at three points around the ring and average them.

5. O-Ring Material Selection Guide

Choosing the wrong O-ring material is the leading cause of seal failure. The material must be compatible with the media being sealed and the operating temperature. Here is the quick-pick guide for the five most common materials:

Buna-N / NBR (Nitrile) Viton / FKM (Fluorocarbon) EPDM (Ethylene Propylene) Silicone / VMQ HNBR (Hydrogenated Nitrile) PTFE (Teflon)

Material	Temp Range	Best For	Avoid With	Cost
Buna-N (NBR) Nitrile, Buna	−40°C to +120°C (−40°F to +248°F)	Petroleum oils, hydraulic fluid, fuel, water, general purpose	Ozone, UV, brake fluid (glycol), ketones, chlorinated solvents	$ Lowest
Viton (FKM) Fluorocarbon	−20°C to +200°C (−4°F to +392°F)	High temp, aggressive chemicals, fuels, aromatic solvents, acids	Ketones, amines, hot water/steam above 120°C, MEK	$$$$ High
EPDM Ethylene Propylene	−40°C to +150°C (−40°F to +302°F)	Hot water, steam, brake fluid, weather/ozone, outdoor exposure	Petroleum oils, fuels, mineral oils, most hydrocarbons	$$ Low–Medium
Silicone (VMQ) Silicon rubber	−55°C to +230°C (−67°F to +446°F)	Food & medical, extreme temps, dry heat, air/gas seals	Petroleum oils, steam, acids, alkalis (poor mechanical strength)	$$$ Medium–High
HNBR Hydrogenated Nitrile	−30°C to +150°C (−22°F to +302°F)	Automotive refrigerants, sour gas, H2S environments, ozone	Strong acids, ketones, esters, aromatic solvents	$$$ Medium–High
PTFE (Teflon) Polytetrafluoroethylene	−200°C to +260°C (−328°F to +500°F)	Universal chemical resistance, corrosive media, low friction	Molten alkali metals, fluorine gas (otherwise near-universal)	$$$$ High

Quick Material Selector: Match Fluid to Material

Fluid / Media	First Choice	Second Choice	Avoid
Engine oil / hydraulic oil	Buna-N (NBR)	HNBR, Viton	EPDM, Silicone
Petrol / diesel / fuel	Buna-N (NBR)	Viton (FKM)	EPDM, Silicone
Water / hot water	EPDM	Silicone, PTFE	Buna-N
Steam	EPDM	PTFE	Buna-N, Viton
Brake fluid (glycol)	EPDM	PTFE	Buna-N, Neoprene
Acids / alkalis (dilute)	Viton (FKM)	EPDM, PTFE	Buna-N, Silicone
Aromatic solvents	Viton (FKM)	PTFE	Buna-N, EPDM, Silicone
Food & beverage contact	Silicone (FDA grade)	EPDM (FDA grade), PTFE	Standard Buna-N
Medical / pharmaceutical	Platinum Silicone	PTFE, FFKM	Most standard rubbers
Refrigerants (R134a etc.)	HNBR	Neoprene	Buna-N (some grades)
Ozone / outdoor / UV	EPDM	Silicone, Viton	Buna-N, Natural rubber
High temp (>120°C)	Viton (FKM)	Silicone, PTFE	Buna-N, EPDM (dynamic)

6. O-Ring Hardness (Durometer)

O-ring hardness is measured in Shore A durometer. The most common hardness for O-rings is 70 Shore A (the industry default). Here is how hardness affects sealing performance:

Shore A	Feel	Pressure Range	Best For
40–50	Very soft / gel-like	Low pressure only	Fragile surfaces, low-load static seals
60	Soft rubber	Low to moderate	Soft surface sealing, vacuum applications
70	Standard rubber	General purpose	Default for most hydraulic, pneumatic, and industrial applications
80–90	Firm rubber	High pressure (hydraulic)	High-pressure dynamic seals, extrusion resistance
90	Hard rubber	Very high pressure	Tube fitting boss seals (AS568 -9xx), hydraulic systems above 3,000 psi

7. O-Ring Groove Design Reference

The O-ring groove (gland) must be designed to allow 10–15% compression of the cross-section for static seals, and 15–25% for dynamic seals. Insufficient compression causes leaks; excessive compression causes extrusion and premature failure.

📌 Groove Depth Formula (Static Face Seal)

Groove Depth = CS × 0.85 to 0.90

This gives 10–15% compression. Example: CS = 3.53 mm → Groove depth = 3.53 × 0.87 ≈ 3.07 mm.
Groove width should be 1.3–1.5 × CS to allow 15–30% volumetric fill. Always consult AS568 or ISO 3601 gland design tables for the exact groove dimensions for your dash number.

8. Frequently Asked Questions

What does the AS568 dash number mean?

The dash number (e.g. -214) is a three-digit identification code from the AS568 standard. The first digit indicates the cross-section series: 0 = 1.78 mm CS, 1 = 2.62 mm CS, 2 = 3.53 mm CS, 3 = 5.33 mm CS, 4 = 6.99 mm CS. The remaining two digits indicate the inside diameter step within that series. So -214 is a 200-series O-ring (CS = 3.53 mm) with the 14th ID step, giving an ID of 24.99 mm (0.984 in).

How do I find my O-ring size?

Measure the Inside Diameter (ID) and Cross-Section (CS) using digital calipers. Match those measurements to the AS568 table (for inch sizes) or the ISO 3601 table (for metric). If your measurements fall exactly on a dash number — done. If they fall between two sizes, check the groove dimensions to determine which size gives proper compression (10–15% for static, 15–25% for dynamic seals).

Can I substitute a metric O-ring for an AS568 size?

Only if the dimensions are close enough to give correct groove compression. Some pairs are near-equivalent: AS568 -0xx (CS = 1.78 mm) vs metric 1.8 mm CS, and AS568 -2xx (CS = 3.53 mm) vs metric 3.55 mm CS. The small difference is usually within tolerance for static seals in non-critical applications. For dynamic seals, aerospace, or high-pressure applications, always use the exact standard the groove was designed for. Never assume interchangeability without verifying groove fill and compression ratio.

What is the most common O-ring size?

In the AS568 standard, the -200 series (CS = 3.53 mm) is the most widely used. Within that series, sizes -210 through -230 cover the most common hydraulic and pneumatic applications. For metric O-rings, the 20 × 2.0 mm and 20 × 3.0 mm designations are among the most frequently specified in European and Asian industrial equipment.

What is the best O-ring material for oil?

Buna-N (NBR / Nitrile) is the standard choice for petroleum oils, hydraulic fluid, and fuel. It is used in approximately 80% of general-purpose sealing applications. For higher temperatures (above 120°C) or aggressive oil blends, upgrade to Viton (FKM). Never use EPDM or Silicone with petroleum-based oils — both will swell, soften, and fail rapidly on contact with mineral oils.

What is the best O-ring material for water and steam?

EPDM is the preferred material for hot water, steam, and water-based fluids. It handles steam up to 150°C and resists hot water better than any other common O-ring elastomer. For very high-pressure steam or temperatures above 150°C, specify PTFE. Never use Buna-N (NBR) with water at elevated temperatures — it degrades and loses elasticity rapidly in hot aqueous environments.

What does Shore A 70 mean for O-rings?

Shore A is the hardness scale for rubber and elastomers. 70 Shore A is the standard O-ring hardness and is the default specification for most AS568 and ISO 3601 O-rings in hydraulic, pneumatic, and general-purpose industrial applications. A 70 Shore A O-ring feels like a firm car tyre sidewall. Higher durometer (80–90 Shore A) is used for high-pressure hydraulic seals and extrusion resistance; lower durometer (50–60 Shore A) is used for soft surface sealing and vacuum applications.

What is the O-ring compression percentage for a static seal?

For static (non-moving) face seals, the recommended cross-section compression is 10–15%. This means the groove depth should be 85–90% of the O-ring's cross-section diameter. For static radial seals (like a shaft passing through a bore), compression is typically 15–20%. For dynamic seals (where the O-ring moves during operation), compression should be 15–25%, but lower compression reduces friction and extends seal life. Always consult the AS568 or ISO 3601 gland design tables for the exact groove dimensions for your specific dash number or metric size.

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