What this calculator does
A silicone calculator like this one turns a run of joints, gaps or expansion cracks into a number of cartridges to buy. Sealant use depends on the cross-section of the bead you lay, not just the length of the joint, so a wide bead in a bath surround uses far more product per metre than a thin bead sealing a window frame.
The figure people get wrong is treating "one cartridge per job" as a rule of thumb. A standard 300 ml cartridge can run anywhere from a few metres to over ten, depending entirely on how wide and deep the bead is laid. Working from the actual bead dimensions and the cartridge size on the tube in your hand gives a real number instead of a guess.
The formula
The volume of sealant used per metre of joint is the bead's cross-sectional area (width times depth) multiplied out over one metre of length. Dividing the cartridge volume by that per-metre figure gives the coverage one cartridge provides at that bead size. The total joint length, increased by the waste allowance to cover tooling-off, false starts and partly-used cartridges, is then divided by that coverage and rounded up to a whole cartridge.
| Term | Meaning |
|---|---|
| Bead width | How wide the sealant line sits across the joint, in millimetres. |
| Bead depth | How deep the sealant fills the joint, in millimetres, roughly the joint gap itself. |
| Cartridge volume | How much sealant one cartridge holds. 300 ml is the common trade size, but sausage packs and smaller tubes vary. |
| Waste allowance | Extra percentage added for tooling off excess, restarting a bead, and cartridges that do not empty completely. |
The inputs explained
| Field | What to enter |
|---|---|
| Total joint length to seal (m) | The combined length of every joint you are sealing, added together if there is more than one run. |
| Bead width (mm) | The width of the sealant bead as it sits on the surface, matched to the gap being sealed. |
| Bead depth (mm) | The depth of sealant filling the joint, often close to the bead width for a standard triangular or square bead. |
| Cartridge volume (standard cartridge ≈ 300 ml, edit for other sizes) (ml) | The volume printed on the cartridge or tube you are using, since sizes vary between products. |
| Waste allowance (%) | A margin for wastage. Ten percent is a reasonable starting point for a straightforward, uninterrupted run. |
When to use it
Sealing a bath or shower surround
A bath edge typically needs a wider, deeper bead than a window frame, since it has to bridge a bigger gap and cope with movement, so it uses noticeably more sealant per metre run.
Pricing a job before buying materials
Working out cartridges needed from the actual joint lengths avoids either running short mid-job or buying several spare cartridges that end up unused.
Comparing a wide bead against a thin one
Doubling the bead width roughly doubles the sealant used per metre, which is easy to underestimate when eyeballing a job rather than measuring it.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How many cartridges for different joint lengths?
A standard 6 mm bead, run out over increasing total joint lengths.
| Total joint length | Cartridges needed | Coverage per cartridge |
|---|---|---|
| 3 m | 1 | 8.3 m |
| 6 m | 1 | 8.3 m |
| 10 m | 2 | 8.3 m |
| 15 m | 2 | 8.3 m |
| 20 m | 3 | 8.3 m |
| 30 m | 4 | 8.3 m |
How does bead width change cartridge use over a fixed run?
The same 10 m joint length sealed at a range of bead widths, with bead depth kept equal to width.
| Bead width (bead depth set equal) | Cartridges needed | Coverage per cartridge |
|---|---|---|
| 3 mm | 1 | 16.7 m |
| 6 mm | 2 | 8.3 m |
| 9 mm | 2 | 5.6 m |
| 12 mm | 3 | 4.2 m |
Questions
What bead depth should I use if the joint depth varies?
Use the deepest point the sealant has to fill along the run, since that is what determines how much product a metre of that joint actually consumes. For a very uneven joint, backer rod is normally used to set a consistent, shallower depth before sealing.
Does a bigger cartridge always work out cheaper?
Usually per millilitre, but only if the job needs enough sealant to use most of it. For a short run, a smaller tube can avoid buying and part-using a large cartridge that then dries out before the next job.
Why increase the length by a waste percentage rather than just rounding cartridges up?
Rounding up cartridges alone assumes none is wasted while applying it, which understates real-world use. The joint won't be tooled off perfectly, some sealant is smeared beyond the joint, and a cartridge rarely empties completely, so a length-based allowance reflects that more realistically.
Is this the same calculation as sealant for tile grout lines?
No. Grout is a different material applied by trowel across the whole tiled area rather than extruded along a single joint, so it is worked out from tile size and joint width instead. See the grout calculator for that job.
For the grout itself between tiles rather than the sealant around the edges, see the tile grout calculator. If the job also involves boards or battens on the same wall, the board and batten calculator covers that material count.