Free tool for builders, precasters & estimators
Expansion Joint Width Calculator
How wide does the joint need to be, and how deep should the sealant go? Enter the panel or slab length, the temperature range and the sealant's movement class — the calculator gives expected thermal movement, minimum joint width and sealant depth. Indicative only: the manufacturer's data sheet and your engineer govern the final design.
Calculate joint width
Melbourne external concrete: roughly 55–70 °C between a winter night and a summer surface in full sun. Internal elements: 15–25 °C.
How the sizing works
Movement = coefficient of thermal expansion × length × temperature range. A 6 m concrete panel over a 60 °C range moves 10 × 10⁻⁶ × 6000 mm × 60 = 3.6 mm in total, so ±1.8 mm about its mid-point.
Joint width = total movement ÷ sealant movement capability. With a ±25% sealant that 3.6 mm of movement needs a joint at least 3.6 ÷ 0.25 ≈ 14.4 mm wide — plus tolerance, because joints are never built exactly to drawing. Real designs also allow for shrinkage, moisture movement and deflection, which is why specified precast joints are commonly 15–25 mm.
Sealant depth ≈ half the width, minimum 6 mm, and rarely more than 12–15 mm. The backer rod sets the depth and stops three-sided adhesion.
Use it, then confirm it. This is thermal movement with typical coefficients; the sealant data sheet, the standard your project is designed to and your engineer govern the final joint. If the joints on site are already narrower than the number above, that's worth a conversation before they're sealed — we can inspect and advise.
Sizing a real job? Precast panel caulking · expansion & control joint sealing · what caulking costs in Melbourne · free quote.
Expansion joint sizing — frequently asked questions
How wide should an expansion joint be?
Wide enough that the sealant can absorb the movement without tearing. Work out the expected movement (material coefficient × length × temperature range), then divide by the sealant's movement capability. A joint sealed with a ±25% sealant must be four times wider than the movement it has to take; with a ±12.5% sealant, eight times wider. Then add construction tolerance — a joint never gets built exactly to the drawing.
How deep should the sealant be in an expansion joint?
About half the joint width, within limits: never less than 6 mm deep, and rarely more than 12–15 mm. A bead deeper than half its width can't stretch properly and tears; shallower than 6 mm hasn't enough body to last. The depth is set with a closed-cell backer rod, which also stops the sealant bonding to the back of the joint.
What is a sealant's movement capability?
The percentage of the joint width the cured sealant can stretch or compress repeatedly without failing — stated on the product data sheet as ±12.5%, ±25% or ±50% (and sometimes as an ISO 11600 class such as 25LM). A ±25% sealant in a 20 mm joint can move 5 mm each way. High-movement polyurethanes and silicones sit at ±25% to ±50%; general fillers are far lower and don't belong in movement joints.
Why do precast panel joints fail even when the width was right?
Usually installation, not design: no backer rod (so the sealant bonds on three sides and tears), depth too great, dirty or unprimed joint faces, or sealing in wet or cold conditions. A correctly sized joint filled the wrong way still fails. See what a proper panel joint looks like on our precast page.
Is this calculator a substitute for engineering design?
No. It gives an indicative width for thermal movement only, using typical material coefficients. Real joints also see moisture movement, creep, shrinkage and structural deflection, and the sealant manufacturer's data sheet and your project engineer govern the final design. Use it to sanity-check a drawing or a quote, then confirm with the specification.