How much caulk do you need? Cartridges from joint length and size

How to work out how much caulk you need

Cartridges are joint length × filled cross-section, plus extra for waste, divided by the volume of one cartridge and rounded up. A 10.1 fl oz cartridge holds about 18.2 cubic inches, so a 1/4 × 1/4 in joint (1/16 sq in) runs about 24 ft per cartridge before waste.

1. Cross-section of a gap. Width × depth of the sealant. Sika asks for a 2:1 width to depth ratio with the sealant between 1/4 and 1/2 in deep. A 1/2 in gap therefore takes 1/4 in of caulk, not more.
2. Backer rod. If the gap is deeper, a closed-cell foam rod goes under the caulk, 1/8 in wider than the gap according to Sika's backer rod sheet. The calculator adds it when there is room for the rod below the caulk; if there is not, it counts the whole depth as caulk.
3. Cross-section of a corner bead. Along a tub or shower pan the bead is a triangle with two equal sides: side × side ÷ 2. A 1/4 in bead is 1/32 sq in.
4. Extra for waste. Data sheet yields are plain geometry: the "up to 55 linear feet" DAP gives for a 10.1 oz cartridge at a 3/16 in round bead is exactly the volume divided by the bead's cross-section. The 10% in the calculator covers the cut nozzle, what stays in the tube and what you wipe off when tooling; change it in Extra for waste.

The advanced options hold the width to depth ratio, the minimum sealant depth and how much wider the backer rod is than the gap.

Worked example: two 3 × 5 ft windows

These are the values the calculator opens with in US units: the perimeter of two windows, with a gap 1/2 in wide and 1 in deep between frame and siding, and 10.1 fl oz cartridges.

• Joint: 2 × (3 + 5) × 2 = 32 ft
• Caulk depth: 1/2 ÷ 2 = 1/4 in, the minimum; 3/4 in is left below, enough for a 5/8 in backer rod
• Cross-section: 1/2 × 1/4 = 1/8 sq in, or 48 cu in of caulk (26.6 fl oz)
• With 10% for waste: 29.3 fl oz
• Cartridges: 29.3 ÷ 10.1 = 2.9, so 3 cartridges of 10.1 fl oz and 32 ft of 5/8 in backer rod

Each cartridge runs 11 ft of this joint. For a corner bead the calculator switches to a 60 × 30 in alcove tub: 10 ft of 1/4 in bead is 2.1 fl oz, 2.3 fl oz with waste. One cartridge is plenty, and so is one 2.8 fl oz squeeze tube.

In the UK the calculator opens in metres: two 120 × 140 cm windows give 10.4 m of joint, 10 mm wide and 20 mm deep. With 5 mm of sealant over a 13 mm backer rod that is 520 ml, 572 ml with waste, so 2 cartridges of 300 ml, each running 5.45 m.

How many feet a 10.1 oz cartridge covers

Feet of joint per 10.1 fl oz cartridge, before waste and with the calculator's 10%. The figures before waste agree with Sika's yield table for the same joints to within 0.1 ft (Sika gives 24.2 ft for 1/4 × 1/4 in and 16.2 ft for 3/8 × 1/4 in).

Joint	Section	Feet	With 10%
Corner bead 1/4 in	1/32 sq in	48.6	44.2
Corner bead 3/8 in	9/128 sq in	21.6	19.6
Gap 1/4 × 1/4 in	1/16 sq in	24.3	22.1
Gap 3/8 × 1/4 in	3/32 sq in	16.2	14.7
Gap 1/2 × 1/4 in	1/8 sq in	12.2	11.0
Gap 3/4 × 3/8 in	9/32 sq in	5.4	4.9
Gap 1 × 1/2 in	1/2 sq in	3.0	2.8

A 2.8 fl oz tube runs a little over a quarter of these lengths: 12.2 ft of 1/4 in corner bead with waste.

Cartridges, squeeze tubes and backer rod

In the US caulk and silicone come in 10.1 fl oz cartridges for a caulking gun (DAP lists theirs as 300 ml, Sika as 299 ml) and in small squeeze tubes such as DAP's 2.8 fl oz. In the UK and Europe cartridges hold 295 to 300 ml (SikaSeal Sanitary Silicone+, Soudal Silirub 1). Enter the size printed on yours.

Each sealant has a usable width. DAP's all-purpose silicone stops at 1/2 in wide joints; Soudal gives 5 to 30 mm for Silirub 1 and Sika 5 to 25 mm for its sanitary silicone. Wider gaps need a construction sealant such as Sikaflex Construction+, rated up to 50 mm.

Backer rod is sold in coils or bags by diameter. Buy it slightly larger than the gap: 1/8 in larger in Sika's sheet, so 5/8 in rod for a 1/2 in gap. It goes in compressed and stays put, and silicone does not stick to polyethylene (Soudal says so in the Silirub 1 sheet).

Common mistakes

• Filling the whole gap. A 1/2 × 1 in gap filled solid takes four times the caulk of a 1/2 × 1/4 in joint and breaks the 2:1 ratio Sika asks for. Use backer rod.
• Three-sided adhesion. The SikaSeal Sanitary Silicone+ sheet asks for adhesion to the two sides only, because bonding to the bottom as well impairs flexibility. Backer rod does that job too.
• Mixing up bead size and width. A 1/4 in corner bead covers 1/4 in on the wall and 1/4 in on the tub. Sika counts 3.6 m per 295 ml for a 9 × 9 mm fillet, the full section of a square; to stay with that figure, choose Joint in a gap and enter 9 × 9 mm.
• Trusting "up to" yields. Data sheet coverage has no waste in it: the cut nozzle, the caulk left in the tube and the excess you tool off are not counted.
• Buying the exact number. Silirub 1 skins in about 15 minutes and has to be tooled before that, so a joint left half done while you fetch another tube will not finish in one piece. Keep the extra for waste and buy everything at once.