Short answer
Multiply the joint width in millimetres by the sealant depth in millimetres by the length in metres. The answer is in millilitres. Divide by the pack volume (280 ml for our silicones, 300 ml for 702 and FR cartridges), add about 10% for waste and round up.
Most short orders come from guesswork. The sums are easy once you know the width and depth of the bead. That means the depth of sealant, not the depth of the gap. Our sealant calculator does the maths for you. Here’s how it works, so you can check the answer.
Measure the bead, not the gap
Width is the distance between the two faces of the joint. Depth is the thickness of sealant you will put in, measured from the face back to the backer rod. A 20 mm gap that is 60 mm deep may only need 10 mm of sealant over a rod.
If you don’t know the depth yet, sort out the joint design first. For silicone, our data sheets give typical depth × width combinations of 6 × 6 mm, 8 × 12 mm and 10 × 20 mm. See joint design basics and when to use backer rod.
If the gap varies along the run, measure it in several places and work out each stretch separately. Don’t size the order from the narrowest reading.
The formula
Width (mm) × depth (mm) × length (m) = millilitres. It works because a metre is 1,000 mm and a millilitre is 1,000 cubic millimetres, so the thousands cancel out.
- 12 mm wide × 8 mm deep × 20 m = 1,920 ml
- 10 mm wide × 10 mm deep × 15 m = 1,500 ml
- 20 mm wide × 10 mm deep × 1 m = 200 ml
That last one is a real figure. It is the application rate recorded for a 20 × 10 mm Pereseal FR joint in its fire test.
Keep the units consistent. If you enter the length in millimetres, divide the answer by 1,000.
Convert to packs
Divide the millilitres by the pack size and round up. Our packs:
- Pereseal 802, N, AF and MP silicones: 280 ml cartridge, 25 per carton
- Pereseal 702 acrylic: 300 ml cartridge (20 per carton), 400 ml sausage (20 per carton) or 600 ml sausage (10 per carton)
- Pereseal FR fire-rated acrylic: 300 ml cartridge, 20 per carton
The 1,920 ml joint above needs 6.9 silicone cartridges, so seven before waste. In 702 it would be 6.4 cartridges of 300 ml, or 3.2 sausages of 600 ml.
Sausages need a sausage gun. On long runs they waste less and leave less rubbish, because there’s no empty cartridge to throw away.
Add for waste
Some sealant always stays in the nozzle, the gun and the end of each pack, and some comes off when you tool. Our calculator starts at 10%. Go higher for short runs with lots of stops and starts, like a bathroom full of small perimeters, or rough masonry where the width wanders. Long, straight, even joints need less.
Work out each product and colour separately. Half a leftover white cartridge is no use on a grey joint.
When the number looks wrong
If the total is huge, check you used the bead depth and not the cavity depth. A missing backer rod is the usual cause. If it looks too small, check you didn’t enter the length in millimetres.
On a big job, compare what the first area actually used with the estimate before you order the rest. If you’re using a lot more, the joints are probably wider or deeper than drawn.
Work it out in the calculator
Enter width, depth, length and pack size. It adds the waste allowance for you.
Open the calculator
