What this calculator works out
This calculator works out the volume of sealant a joint will take from its length, width and depth, adds a waste allowance, and returns the number of cartridges to buy.
The unit arithmetic works out neatly: a joint measured in metres, with width and depth in millimetres, gives a volume directly in millilitres. One metre of a 6 by 6 mm joint is 36 ml.
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Before you order
- Measure the joint width and depth as they will be after preparation, not as the gap currently looks.
- A standard cartridge is 300 ml. Larger 400 and 600 ml sizes exist for foil packs and skeleton guns.
- Allow at least 15% waste. Sealant is wasted at the start of each cartridge, in tooling off, and in the tail left when the gun cannot push more out.
- For deep joints, fit backer rod first so the sealant does not bond to the back of the gap.
A sealant joint should be about twice as wide as it is deep, and should bond to two faces only, not three. A joint bonded on three sides cannot flex and will tear as the gap moves. Backer rod or bond breaker tape at the back of a deep joint is what makes the difference between a seal that lasts years and one that fails in a winter.
How this calculator works
A volume, an allowance, then cartridges:
Volume (ml) = joint length in m × width in mm × depth in mm
With waste = volume × (1 + waste ÷ 100)
Cartridges = volume with waste ÷ cartridge volume, rounded upThe unit conversion is exact rather than approximate: one metre by one millimetre by one millimetre is 1,000 mm³, which is one millilitre. No conversion factor is needed.
Worked example: 20 m of 6 mm joint
Using the default figures — 20 m of joint at 6 mm wide and 6 mm deep, 300 ml cartridges, 15% waste:
- Volume: 20 × 6 × 6 = 720 ml
- With waste: 828 ml
- Cartridges: 3
Note that this joint is square in section rather than the recommended two-to-one ratio. Widening it to 12 mm and keeping the depth at 6 would double the volume to 1,440 ml and need five cartridges — but would produce a joint that can actually accommodate movement. The cheaper option is frequently the one that fails first.
Common mistakes
- Making the joint as deep as it is wide. Aim for roughly twice as wide as deep.
- Allowing three-sided adhesion. Use backer rod so the sealant bonds to two faces only.
- Applying over a damp or dusty surface. Adhesion fails and the joint peels away.
- Using the wrong type. Silicone will not accept paint; acrylic is paintable but less flexible and less water-resistant.
- Cutting the nozzle too wide. Cut small first — you can always enlarge it.
Frequently asked questions
Silicone or acrylic?
Silicone is flexible and water-resistant, which suits baths, showers, sinks and external joints, but it cannot be painted. Acrylic decorator's caulk is paintable and suits internal joints around skirting, architrave and cornice, but it is less flexible and unsuitable where water sits. Using the wrong one is the most common sealant mistake.
How do I get a neat finish?
Mask both sides of the joint with tape, apply a continuous bead in one pass, tool it smooth with a wet finger or a shaping tool, then remove the tape immediately while the sealant is still wet. Removing tape after skinning tears the edge. Working in shorter runs makes a continuous bead much easier.
Do I need to remove the old sealant?
Yes, all of it. New sealant will not bond reliably to old, particularly to silicone, and any mould behind the old bead will grow through. Cut it out with a sharp blade, clean the joint with a suitable solvent, and let it dry completely before applying new.
Why does my bathroom sealant keep going black?
Mould growing in the sealant, usually because the room is not ventilated enough for the moisture it produces. Mould-resistant sealants delay it rather than prevent it. Running an extractor for 15 to 20 minutes after showering, and wiping the joint dry, does more than any product choice.
Is what I enter stored?
No. Joint measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Health and Safety Executive — safe handling of sealants and solvents, including ventilation
- GOV.UK — building regulations guidance on ventilation and moisture in dwellings
Sources are checked at publication and can change — how I choose and check references.