Sealant Calculator

How much sealant do I need?

Sealant is a volume calculation. The bead has a cross-section, measured in square millimetres, and it runs for a length, measured in metres. Multiply the two and you have the volume, and the arithmetic is kind: a bead of 1 mm² running 1 m is 1,000 mm³, which is exactly 1 ml. So a 6 x 6 mm triangular fillet (18 mm²) uses 18 ml per metre, and a 10 x 8 mm rectangular gap (80 mm²) uses 80 ml per metre. Divide the cartridge size by that figure and you have the metres per tube.

The bit people miss is the allowance. Sealant is never applied at the theoretical section: the bead goes on fatter than planned, each start and stop wastes a blob, tooling pushes some off, and 10 to 20 ml always stays behind in the nozzle and the base of the cartridge. On a bath you might use a third of a tube where the sum says a fifth. That is why the default allowance here is 25%, and why a first-timer on a job with many short runs should choose 40%.

The formula

How far one cartridge goes

Retailers often quote "up to 12 metres per tube" without saying what bead they mean. As the table shows, the true figure runs from 2 metres to 50 depending on the joint, so measure the gap before you trust the label.

Worked example

A bathroom refit: a bath (3.2 m), a shower tray (2.4 m), a basin and a worktop edge (1.2 m), and the top of the tiled splashback (3.2 m), which adds up to 10 m of joint. All of it is a 6 x 6 mm triangular fillet.

Section = 6 × 6 ÷ 2 = 18 mm², so 18 ml per metre. Net volume = 18 × 10 = 180 ml. Add the 25% allowance = 225 ml. One 310 ml cartridge covers 310 ÷ (18 × 1.25) = 13.8 m at that rate, so the answer is 1 cartridge, with about 85 ml spare for the inevitable redo of one corner.

Change the same 10 m to a 10 x 8 mm rectangular gap around window frames and the volume jumps to 80 ml per metre: 800 ml net, 1,000 ml with allowance, which is 4 cartridges. The joint shape matters far more than the length.

Which sealant for which joint

• Sanitary silicone (with a fungicide) for baths, showers, basins and the joint between tiles and worktop. Use a colour that matches the grout or the fitting, not the tile.
• Decorator caulk (acrylic) for the gap above skirting, around architraves and any joint you intend to paint. It is not waterproof and it cracks if the gap moves much.
• Frame sealant (low-modulus silicone or hybrid polymer) for window and door frames outside, where the joint moves with temperature and the sealant has to stretch.
• Expansion joint sealant (polyurethane or hybrid) for movement joints in floors and paving, always over a backer rod so it can stretch from both faces.
• Heated appliances and flues: use only the joint material specified for the exact appliance or flue system. A high-temperature label alone does not establish suitability; check the permitted substrates and continuous and short-term temperature limits.

Getting a clean bead

1. Cut the nozzle at 45 degrees with an opening slightly smaller than the bead you want. You can always add; you cannot take away.
2. Mask both sides of the joint with tape for a straight line. Pull the tape immediately after tooling, before the sealant skins.
3. Push the gun away from you along the joint rather than dragging it, so the nozzle forces sealant into the gap instead of laying it on top.
4. Tool with a proper profiling tool or a wetted finger in one continuous stroke. Stopping and starting mid-run leaves a ridge.
5. For a bath, fill it with water first and seal it full. The bath drops a millimetre or two under load; a bead applied empty is put into tension every time you bathe and fails early.

Planning your order

Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.