O-Ring Size Chart (AS568)
An AS568 O-ring is described by three numbers: inside diameter, cross section and dash number. Get any two and you have the part. This page gives you the full dash number tables for the four series that cover almost everything in hydraulics, plus how to measure an O-ring you already have in your hand.
How to measure an O-ring
- Cross section. Measure the thickness of the ring with calipers or a comparator. Do not squeeze it — rubber compresses and you will read low. This gives you the series.
- Inside diameter. Lay the ring flat and measure across the hole, or slide it onto a cone gauge. Never stretch it to measure.
- Match the two. Find your cross section series below, then find the closest inside diameter. That is your dash number.
A used O-ring that has been under pressure and heat is usually swollen or flattened, so treat a measurement off an old seal as a starting point. If it has taken a permanent set, measure the groove instead.
Cross section series at a glance
| Series | Cross section (in) | Cross section (mm) | Dash number range |
|---|---|---|---|
| 1/16″ | .070 ± .003 | 1.78 | -004 to -050 |
| 3/32″ | .103 ± .003 | 2.62 | -102 to -150 |
| 1/8″ | .139 ± .004 | 3.53 | -201 to -284 |
| 3/16″ | .210 ± .005 | 5.33 | -309 to -395 |
| 1/4″ | .275 ± .006 | 6.99 | -425 to -475 |
1/16″ cross section — .070″
| AS568 dash no. | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 004 | .070 | 1.78 |
| 005 | .101 | 2.57 |
| 006 | .114 | 2.90 |
| 007 | .145 | 3.68 |
| 008 | .176 | 4.47 |
| 009 | .208 | 5.28 |
| 010 | .239 | 6.07 |
| 011 | .301 | 7.65 |
| 012 | .363 | 9.23 |
| 013 | .426 | 10.82 |
| 014 | .488 | 12.41 |
| 015 | .551 | 14.00 |
| 016 | .613 | 15.58 |
| 017 | .676 | 17.17 |
| 018 | .738 | 18.76 |
| 019 | .801 | 20.35 |
| 020 | .863 | 21.93 |
| 021 | .926 | 23.52 |
| 022 | .988 | 25.11 |
| 023 | 1.051 | 26.70 |
| 024 | 1.113 | 28.28 |
| 025 | 1.176 | 29.87 |
| 026 | 1.238 | 31.46 |
| 027 | 1.301 | 33.05 |
| 028 | 1.363 | 34.63 |
| 029 | 1.426 | 36.22 |
| 030 | 1.551 | 39.40 |
| 031 | 1.676 | 42.57 |
| 032 | 1.801 | 45.75 |
| 033 | 1.926 | 48.92 |
| 034 | 2.051 | 52.10 |
| 035 | 2.176 | 55.27 |
| 036 | 2.301 | 58.45 |
| 037 | 2.426 | 61.62 |
| 038 | 2.551 | 64.80 |
| 039 | 2.676 | 67.97 |
| 040 | 2.801 | 71.15 |
| 041 | 2.926 | 74.32 |
| 042 | 3.051 | 77.50 |
| 043 | 3.301 | 83.85 |
| 044 | 3.551 | 90.20 |
| 045 | 3.801 | 96.55 |
| 046 | 4.051 | 102.90 |
| 047 | 4.301 | 109.25 |
| 048 | 4.551 | 115.60 |
| 049 | 4.801 | 121.95 |
| 050 | 5.051 | 128.30 |
3/32″ cross section — .103″
| AS568 dash no. | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 102 | .049 | 1.24 |
| 103 | .080 | 2.04 |
| 104 | .112 | 2.83 |
| 105 | .143 | 3.63 |
| 106 | .174 | 4.42 |
| 107 | .205 | 5.21 |
| 108 | .236 | 6.01 |
| 109 | .268 | 6.80 |
| 110 | .299 | 7.59 |
| 111 | .330 | 8.39 |
| 112 | .361 | 9.18 |
| 113 | .393 | 9.98 |
| 114 | .424 | 10.77 |
| 115 | .455 | 11.56 |
| 116 | .486 | 12.36 |
| 117 | .518 | 13.15 |
| 118 | .549 | 13.94 |
| 119 | .580 | 14.74 |
| 120 | .611 | 15.53 |
| 121 | .643 | 16.33 |
| 122 | .674 | 17.12 |
| 123 | .705 | 17.91 |
| 124 | .736 | 18.71 |
| 125 | .768 | 19.50 |
| 126 | .799 | 20.29 |
| 127 | .830 | 21.09 |
| 128 | .861 | 21.88 |
| 129 | .893 | 22.68 |
| 130 | .924 | 23.47 |
| 131 | .955 | 24.26 |
| 132 | .986 | 25.06 |
| 133 | 1.018 | 25.85 |
| 134 | 1.049 | 26.64 |
| 135 | 1.080 | 27.44 |
| 136 | 1.111 | 28.23 |
| 137 | 1.143 | 29.03 |
| 138 | 1.174 | 29.82 |
| 139 | 1.205 | 30.61 |
| 140 | 1.236 | 31.41 |
| 141 | 1.268 | 32.20 |
| 142 | 1.299 | 32.99 |
| 143 | 1.330 | 33.79 |
| 144 | 1.361 | 34.58 |
| 145 | 1.393 | 35.38 |
| 146 | 1.424 | 36.17 |
| 147 | 1.455 | 36.96 |
| 148 | 1.486 | 37.76 |
| 149 | 1.518 | 38.55 |
| 150 | 1.549 | 39.34 |
1/8″ cross section — .139″
| AS568 dash no. | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 201 | .171 | 4.34 |
| 202 | .234 | 5.93 |
| 203 | .296 | 7.52 |
| 204 | .359 | 9.11 |
| 205 | .421 | 10.69 |
| 206 | .484 | 12.28 |
| 207 | .546 | 13.87 |
| 208 | .609 | 15.46 |
| 209 | .671 | 17.04 |
| 210 | .734 | 18.63 |
| 211 | .796 | 20.22 |
| 212 | .859 | 21.81 |
| 213 | .921 | 23.39 |
| 214 | .984 | 24.98 |
| 215 | 1.046 | 26.57 |
| 216 | 1.109 | 28.16 |
| 217 | 1.171 | 29.74 |
| 218 | 1.234 | 31.33 |
| 219 | 1.296 | 32.92 |
| 220 | 1.359 | 34.51 |
| 221 | 1.421 | 36.09 |
| 222 | 1.484 | 37.68 |
| 223 | 1.546 | 39.27 |
| 224 | 1.671 | 42.44 |
| 225 | 1.796 | 45.62 |
| 226 | 1.921 | 48.79 |
| 227 | 2.046 | 51.97 |
| 228 | 2.171 | 55.14 |
| 229 | 2.296 | 58.32 |
| 230 | 2.421 | 61.49 |
| 231 | 2.546 | 64.67 |
| 232 | 2.671 | 67.84 |
| 233 | 2.796 | 71.02 |
| 234 | 2.921 | 74.19 |
| 235 | 3.046 | 77.37 |
| 236 | 3.171 | 80.54 |
| 237 | 3.296 | 83.72 |
| 238 | 3.421 | 86.89 |
| 239 | 3.546 | 90.07 |
| 240 | 3.671 | 93.24 |
| 241 | 3.796 | 96.42 |
| 242 | 3.921 | 99.59 |
| 243 | 4.046 | 102.77 |
| 244 | 4.171 | 105.94 |
| 245 | 4.296 | 109.12 |
| 246 | 4.421 | 112.29 |
| 247 | 4.546 | 115.47 |
| 248 | 4.671 | 118.64 |
| 249 | 4.796 | 121.82 |
| 250 | 4.921 | 124.99 |
The 1/8″ series continues past -250 in quarter inch steps up to -284. Contact us for those sizes.
3/16″ cross section — .210″
| AS568 dash no. | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 309 | .412 | 10.46 |
| 310 | .474 | 12.05 |
| 311 | .537 | 13.64 |
| 312 | .599 | 15.23 |
| 313 | .662 | 16.81 |
| 314 | .724 | 18.40 |
| 315 | .787 | 19.99 |
| 316 | .849 | 21.58 |
| 317 | .912 | 23.16 |
| 318 | .974 | 24.75 |
| 319 | 1.037 | 26.34 |
| 320 | 1.099 | 27.93 |
| 321 | 1.162 | 29.51 |
| 322 | 1.224 | 31.10 |
| 323 | 1.287 | 32.69 |
| 324 | 1.349 | 34.28 |
| 325 | 1.412 | 35.86 |
| 326 | 1.537 | 39.04 |
| 327 | 1.662 | 42.21 |
| 328 | 1.787 | 45.39 |
| 329 | 1.912 | 48.56 |
| 330 | 2.037 | 51.74 |
| 331 | 2.162 | 54.91 |
| 332 | 2.287 | 58.09 |
| 333 | 2.412 | 61.26 |
| 334 | 2.537 | 64.44 |
| 335 | 2.662 | 67.61 |
| 336 | 2.787 | 70.79 |
| 337 | 2.912 | 73.96 |
| 338 | 3.037 | 77.14 |
| 339 | 3.162 | 80.31 |
| 340 | 3.287 | 83.49 |
| 341 | 3.412 | 86.66 |
| 342 | 3.537 | 89.84 |
| 343 | 3.662 | 93.01 |
| 344 | 3.787 | 96.19 |
| 345 | 3.912 | 99.36 |
| 346 | 4.037 | 102.54 |
| 347 | 4.162 | 105.71 |
| 348 | 4.287 | 108.89 |
| 349 | 4.412 | 112.06 |
The 3/16″ series continues past -349 up to -395. Contact us for those sizes.
1/4″ cross section — .275″
| AS568 dash no. | Inside diameter (in) | Inside diameter (mm) |
|---|---|---|
| 425 | 4.475 | 113.66 |
| 426 | 4.600 | 116.84 |
| 427 | 4.725 | 120.01 |
| 428 | 4.850 | 123.19 |
| 429 | 4.975 | 126.36 |
| 430 | 5.100 | 129.54 |
| 431 | 5.225 | 132.71 |
| 432 | 5.350 | 135.89 |
| 433 | 5.475 | 139.06 |
| 434 | 5.600 | 142.24 |
The 1/4″ series runs from -425 to -475 and covers the large diameters used on cylinder heads and pump housings. We can supply the full range — call with the diameter you measured.
A note on metric O-rings
Metric O-rings are not part of AS568 and are called out directly as inside diameter by cross section, for example 20 x 2.5 mm. They are common on European and Japanese equipment. An AS568 size will sometimes be close enough in diameter to fit a metric groove, but the cross section rarely matches, and cross section is what actually does the sealing. Do not substitute across systems on a pressure joint.
Choosing a material
Nitrile (Buna-N) handles petroleum hydraulic oil and is the default for most hydraulic work. Viton (FKM) is the choice for higher temperatures and more aggressive fluids. Polyurethane is used where extrusion resistance matters. If the ring you pulled out was a different colour than you expected, it may have been a deliberate material choice rather than a mistake — check before you swap it.
Dimensions here are nominal AS568 values given for identification. Tolerances vary by size and material; confirm against your application before ordering.
Shop O-rings and seals
We carry O-rings, O-ring assortment kits, rod and piston seals, wipers, wear rings and complete hydraulic seal kits. If you are rebuilding a cylinder, the seal measurement guide will help you spec the whole stack. Also see the thread identification chart and the rest of our reference charts.