Engineering tools
O-ring calculators
Five quick calculations for everyday O-ring work: cord length for spliced rings, inside diameter from a cord, installed stretch, estimated weight and unit conversion. Every result links to the sizes that fit.
Cord length for a spliced O-ring
Length of round cord needed for one ring, measured on the centreline.
L = π × (ID + CS) — the centreline circumference. Add a cutting allowance for splicing.
Inside diameter from a cord length
What ID a cord of a given length makes when joined into a ring.
ID = L ÷ π − CS
Installed stretch or compression
How much the ring is stretched onto a groove or rod, or compressed into a bore.
Stretch % = (D − ID) ÷ ID × 100. A stretched ring’s cross section gets thinner; the estimate uses CS reduction ≈ half the stretch.
Estimated weight
For shipping and planning. Uses a typical density for the material family.
V = π² ÷ 4 × CS² × (ID + CS), weight = V × density. Densities are typical values for the family; the actual compound varies.
Millimetres ⇄ inches
Convert a size and check the nearest AS568 dash number.
Using the results
Calculator questions
Results are calculated from nominal dimensions. They support planning and do not replace a gland design review.
How much cord do I need for a spliced O-ring?
Multiply π by the inside diameter plus the cross section: L = π × (ID + CS). That is the centreline length of the finished ring. Add a small allowance for cutting and joining; we confirm the exact allowance when we make spliced rings.
How much stretch is acceptable?
As a common reference, installed stretch on the inside diameter is kept to about 5% or less. More stretch thins the cross section and reduces squeeze. Use the groove design tool to check squeeze and gland fill together.
Why is the weight only an estimate?
Density varies with the compound and filler system. The calculator uses a typical value for each material family, so the result is suitable for shipping and planning, not for quality checks.
Does a converted size equal an AS568 size?
Not necessarily. The converter shows the nearest AS568 dash number and the difference. Close is not the same as equal; compare with the tolerances before substituting.