O Ring Size Calculator matching caliper readings to the closest AS568 dash number, then checking squeeze, stretch and gland fill for that ring in a piston, rod or face seal groove.
Caliper a new or unused ring if you can. A worn ring shrinks in section and grows in diameter, which can point to the wrong size.
Groove Width, Service and Pressure
Swell is the volume gain the fluid causes, from the compound’s data sheet. Leave it at 0% if you don’t know it.
Closest AS568 Sizes
| Dash | ID | Cross-Section | OD | Your Ring |
|---|---|---|---|---|
| -214 | 0.984 in | 0.139 in | 1.262 in | In Tolerance |
| -213 | 0.921 in | 0.139 in | 1.199 in | Outside |
| -215 | 1.046 in | 0.139 in | 1.324 in | Outside |
| -216 | 1.109 in | 0.139 in | 1.387 in | Outside |
| -913 | 0.986 in | 0.116 in | 1.218 in | Outside |
| -212 | 0.859 in | 0.139 in | 1.137 in | Outside |
| -912 | 0.924 in | 0.116 in | 1.156 in | Outside |
| -914 | 1.047 in | 0.116 in | 1.279 in | Outside |
O Ring Size Calculator for AS568 Dash Numbers
The O Ring Size Calculator does two jobs that usually live on separate charts. It turns a caliper reading of an old ring into the closest AS568 dash number, and it checks whether a chosen ring will seal in a groove you have measured. Mechanics use the first mode at the parts counter, and anyone machining or reusing a gland uses the second.
Both modes work in inches and psi or in millimeters and bar, converted at 25.4 mm per inch. Identify mode needs the ring’s inside or outside diameter and its cross-section. Groove mode needs the ring, the gland type, the bore or rod size, the groove diameter and groove width, plus pressure and any expected fluid swell.
Reading a Ring Back to Its Dash Number
AS568 sizes are named by a dash number, and the first digit mostly tells you the cross-section family. The 0 series is 0.070 in, 100 is 0.103 in, 200 is 0.139 in, 300 is 0.210 in and 400 is 0.275 in, while the 900 series covers straight-thread tube fitting boss seals. The calculator compares your readings with every size and its published tolerance, then ranks the closest matches.
A ring measured at 0.980 in inside diameter and 0.138 in cross-section comes back as -214. That size is 0.984 in by 0.139 in with a tolerance of ±0.010 in on the diameter and ±0.004 in on the section, so both readings pass. In metric it is about 24.99 by 3.53 mm, and the card also shows the next closest size in case your ring sits between two.
A common mistake is measuring a ring that has been in service for years. Heat and compression flatten the section over time, so a used ring can read thinner than new, and a stretched ring can read larger in diameter. The calculator flags that pattern as a possibly worn ring rather than forcing a match, so check the groove when the readings look off.
If you need to splice a ring from cord stock, the last card gives the cut length. It uses the mean diameter, the line through the middle of the section, because that line keeps its length when the cord is bent into a ring.
$$\text{Cord length} = \pi \times (\text{ID} + \text{Cross-section})$$
For the -214 that is about 3.528 in of cord before bonding. The inside and outside circumferences sit beside it on the card, which helps when you check an existing ring with a tape.
Checking an O-Ring in a Groove
Groove mode works out three numbers that decide whether the seal holds. Squeeze is how much the groove compresses the section, stretch is how much the ring grows to sit on a piston, and gland fill is how much of the groove the ring occupies. Each one has its own safe window in the design guides from seal makers.
$$\text{Squeeze} = \frac{\text{Cross-section} – \text{Gland depth}}{\text{Cross-section}} \times 100$$
$$\text{Gland fill} = \frac{\pi \times W^2 / 4}{\text{Gland depth} \times \text{Groove width}} \times 100$$
For a piston, the gland depth is half the difference between the bore and the groove diameter. For a rod gland, it is half the difference between the groove diameter and the rod. For a face seal, you enter the groove depth directly, and the calculator also checks that the ring’s outside diameter reaches the groove wall.
Why Stretch Changes the Squeeze
Stretching a ring onto a piston thins its section, so the real squeeze is lower than the free section suggests. Parker’s static seal design guide says that past two or three percent stretch, the squeeze should be applied to the stretched section. The O Ring Size Calculator does that automatically, estimating the thinner section by holding the ring’s volume constant.
The default case shows the difference. A -214 on a 1.030 in groove in a 1.250 in bore is stretched about 4.7%, which thins the section from 0.139 to about 0.136 in. The squeeze works out to 19.0% on the stretched section, against 20.9% if stretch were ignored.
The O-ring design guide from Zatkoff Seals caps installed stretch at 5%, because more stretch shortens the life of most compounds. The calculator warns above 5% and treats 8% as too much. Parker also suggests that very large piston rings can go one size smaller, since the design stretch is so small that the ring sags out of the groove during assembly.
Squeeze Bands, Fill and Pressure
The calculator targets 15 to 30% squeeze for static radial seals, 10 to 20% for sliding seals and 20 to 32% for face seals. Published ranges vary a little between makers, so treat the band as a guide rather than a hard rule. The compression card shows how much deeper or shallower the groove would need to be to land in the middle of the band.
Gland fill should stay under about 85%, and Wayne Rubber puts the working range at 65 to 85%. Room is needed for heat and fluid swell, so the calculator adds your swell percentage and flags a groove that would overfill. The default -214 groove fills about 70%, well inside that range.
Pressure matters for the gap behind the ring. Parker’s static gland table is rated to 1,500 psi (103.5 bar), and above that it points to wider grooves that allow backup rings. The calculator suggests backup rings above that pressure, since the ring can be pushed into the clearance gap.
The last card searches every AS568 size for one that fits the groove with squeeze in band, fill under 85% and stretch between 0 and 5%. When none qualifies, it reports the nearest by squeeze and says the groove itself needs to change.
Ordering the Right Ring
A dash number sets only the size. Order it with the compound as well, such as 70 durometer nitrile or FKM chosen for your fluid and temperature, because the dash number says nothing about material. If no AS568 size fits your readings, the ring may be metric, and ISO 3601 or JIS charts cover sections like 2.5 or 3 mm that AS568 does not.