Two numbers decide it
The cartridges of caulk, sealant or filler you need depend on two numbers: the total length of the joints, and the size of the bead you run along them. The amount is length × cross-section of the bead. The volume of one cartridge is printed on its label, usually in fluid ounces in the US and in milliliters elsewhere.
- Triangular fillet, in a corner between two surfaces, such as the top of a baseboard against the wall or a bath against the tiles: cross-section = width × width ÷ 2, where the width is how far the bead reaches along each surface.
- Rectangular joint, in a gap between two edges: cross-section = width × depth.
One handy fact: 1 mm² of cross-section over 1 m of joint is exactly 1 mL. A 6 mm fillet is 6 × 6 ÷ 2 = 18 mm², so it uses 18 mL of sealant per meter, and a 300 mL cartridge gives 300 ÷ 18 = 16.7 m of bead.
How far one cartridge goes
Because the width is multiplied by itself, the bead size changes the result far more than anything else: halve the width and one cartridge goes four times as far. For a triangular fillet, before any allowance:
| Fillet width | Sealant per meter | One 300 mL cartridge |
|---|---|---|
| 3 mm | 4.5 mL | 66.7 m |
| 5 mm | 12.5 mL | 24 m |
| 6 mm | 18 mL | 16.7 m |
| 8 mm | 32 mL | 9.4 m |
| 10 mm | 50 mL | 6 m |
| Fillet width | One 10.1 fl oz cartridge |
|---|---|
| 1/8 in | 194 ft |
| 3/16 in | 86 ft |
| 1/4 in | 48.6 ft |
| 3/8 in | 21.6 ft |
| 1/2 in | 12.2 ft |
One US fluid ounce is exactly 29.5735295625 mL, so 10.1 fl oz is 298.7 mL. Both cartridge sizes are examples: use the volume printed on yours.
Worked example: one room, in feet and inches
The room the room refresh planner starts with: 12 × 10 ft, one 32 × 80 in door and one 36 × 48 in window. The bead is a 1/4 in fillet, the cartridges hold 10.1 fl oz, and the allowance is 10%.
- Top of the baseboard: the perimeter minus the door, 44 − 2.67 = 41.33 ft (12.6 m). At 20.16 mL per meter that is 254 mL, or 279.4 mL with the allowance: 1 cartridge, just under the 298.7 mL it holds.
- Around the door and the window: the door gives 2 × 80 + 32 = 192 in and the window 2 × 48 + 2 × 36 = 168 in, so 360 in, which is 30 ft and another 184.4 mL.
- Both together: 438.4 mL, or 482.2 mL with the allowance, which is 1.61 cartridges: 2 cartridges, with about 115 mL left over.
The same room with a 3/8 in bead along the baseboard needs 571.5 mL, or 628.6 mL with the allowance: 3 cartridges for that one line, where the 1/4 in bead needed one.
Worked example: one room, in metric
The metric worked example on the room refresh page is a room of 4.2 × 3.5 m with one 0.8 m door, so the baseboard runs 15.4 − 0.8 = 14.6 m. With 300 mL cartridges and a 10% allowance:
- A 6 mm fillet: 14.6 × 18 = 262.8 mL, or 289.1 mL with the allowance, so 1 cartridge.
- An 8 mm fillet: 14.6 × 32 = 467.2 mL, or 513.9 mL with the allowance, so 2 cartridges.
Two millimeters of bead width is the difference between one cartridge and two.
Worked example: a bath and a gap
- A bath 1.7 m long and 0.7 m wide, against the wall along one side and one end: 1.7 + 0.7 = 2.4 m of bead. A 6 mm fillet takes 2.4 × 18 = 43.2 mL, or 47.5 mL with the allowance: one cartridge, of which you use about a sixth.
- A gap 5 mm wide along a 3 m worktop, filled 5 mm deep: 5 × 5 = 25 mL per meter, so 75 mL, or 82.5 mL with the allowance. Filled twice as deep, it needs twice as much.
Common mistakes
- Measuring across the face of the bead. A fillet that reaches 6 mm along each surface has a sloping face about 8.5 mm wide. Typed as 8.5 mm, it becomes 36.1 mL per meter instead of 18: twice the sealant.
- Leaving out the second side of a door. The bead around a door is counted per room. A door between two rooms has a frame on each side: to seal both, the door has to be in the openings of both rooms, with the bead chosen in both.
- Rounding each line on its own. Lines of the same cartridge are added before rounding, as how pack rounding works explains. In the feet example the two lines need 279.4 mL and 202.8 mL with the allowance, 482.2 mL together. That is 2 cartridges either way, but on a longer job adding first can save one.
- Planning without any allowance. Some sealant stays in the nozzle and at the end of the cartridge, and a gap can be deeper in places than it looks. The planner starts at 10%, which you can change.
Where the planner counts beads
In step 2 of the room refresh planner or any of the indoor calculators, open Plan other materials, then Sealant and filler, add your cartridge with its volume and the bead size, and choose for each room where it goes: along the top of the baseboard, around doors and windows, along the top edge of the wall tiles or the edge of a tiled floor in the tile calculator, along the paneling in the board and batten calculator, along both edges of coving, or a length you type in. The baseboard and the openings come from the room measurements, as in how to measure a room.
The planner only counts. Choose a sealant suited to the joint and the room, for example a bathroom, and follow the cartridge’s instructions for preparing the joint, the largest gap it fills and the time it needs to cure.