About this calculator
This calculator works out how many cartridges or foil packs of sealant (silicone, hybrid/MS polymer, polyurethane, acrylic) a job needs, and how much backer rod.
List every joint with its length and cross-section. A rectangular joint is a gap between two surfaces — a movement joint in a floor or façade, a window perimeter — filled to a set depth, usually over a backer rod. A triangular fillet is a corner bead, such as a bath, shower tray or worktop against a wall; its width and depth are the two legs of the triangle.
The sealant volume is each joint's cross-section times its length, plus a waste allowance; cartridges and packs are rounded up. For rectangular joints the calculator also sizes the backer rod.
Formula
rectangular joint volume = width × depth × length
triangular fillet volume = width × depth / 2 × length
sealant needed = Σ volumes × (1 + waste %)
packs = ceil(sealant needed / pack volume) (1 ml = 1000 mm³)
backer rod diameter = ceil(1.25 × joint width) (rule of thumb)
backer rod length = length of the rectangular joints, per diameter
Worked example
10 m of rectangular joint 10 mm wide and 6 mm deep, 5 m of 6 × 6 mm fillet, 310 ml cartridges, 10 % waste:
rectangular = 10 × 6 × 10 000 = 600 000 mm³ = 600 ml
fillet = 6 × 6 / 2 × 5000 = 90 000 mm³ = 90 ml
with waste = 690 × 1.10 = 759 ml
cartridges = ceil(759 / 310) = ceil(2.45) = 3
backer rod = ceil(1.25 × 10) = 13 mm diameter, 10 m
FAQ
How deep should the sealant be? It depends on the product and the joint's movement; the data sheet gives it. As examples, Soudal advises a 6 mm depth for joints up to 12 mm wide and half the width above that, and Sika requires a 2:1 width-to-depth ratio for one of its fire-rated silicones. The calculator warns when a rectangular joint is deeper than it is wide.
Why use a backer rod? It sets the sealant depth, saves sealant and stops the sealant sticking to the back of the joint (three-sided adhesion), so the bead can stretch. Closed-cell rod is pushed in slightly compressed.
How is the backer rod size chosen? About 25 % larger than the joint width — Sika states "approx. 25% larger than the joint width" for its backing rod and 25–30 % in its New Zealand sealant guide. This is a manufacturer rule of thumb, not a standard. Buy the nearest stocked size above the calculated diameter.
Which standard does this follow? None. ISO 11600 and EN 15651 classify sealant products by movement capability and use; they do not give a consumption method, so they are not cited.
What does the waste allowance cover? The nozzle cut-off, tooling the bead, and what stays in the cartridge or gun. 10 % is a starting point, not a norm.
Assumptions and limits
No standard is cited: the volume is plain geometry, and the backer rod rule is a manufacturer rule of thumb.
Defaults to replace: the two example joints, 310 ml cartridges, 10 % waste.
Geometry: a rectangular joint is filled flat to the given depth (a concave tooled surface uses slightly less); a fillet is a right-angled triangle with the given legs. Surface preparation, primer, masking tape and bond-breaker tape are not counted. The backer rod drawn in the cross-section is schematic.