Hong Life CalculatorsUnderstanding the caulk and sealant calculator — concept and principle
This calculator estimates how many cartridges of caulk or sealant you need for a joint of a given length, width and depth. Use it for sealing around baths, windows, door frames, skirting, expansion joints or facade joints, so you buy enough cartridges to finish without a trip back to the shop.
How it is calculated
- Volume (mL) = joint length (m) × width (mm) × depth (mm).
- Cartridges = ⌈volume ÷ cartridge volume × 1.1⌉ (10% for waste and nozzle loss).
The units combine neatly: 1 m × 1 mm × 1 mm = 1,000 mm × 1 mm² = 1,000 mm³ = 1 mL.
The principle
A sealant joint filled to a rectangular cross-section is a long, thin box, so its volume is length × width × depth. Using metres for length and millimetres for the cross-section gives the answer directly in millilitres, which matches how cartridges are labelled. The 10% allowance covers sealant left in the nozzle and cartridge, tooling, and small variations in joint size.
Worked example
Default inputs: joint 20 m long, 6 mm wide, 6 mm deep, 300 mL cartridges.
- Volume = 20 × 6 × 6 = 720 mL.
- With waste = 720 × 1.1 = 792 mL.
- Cartridges = 792 ÷ 300 = 2.64, rounded up to 3 cartridges.
| Cartridge volume (mL) | Cartridges |
|---|---|
| 75 | 11 |
| 150 | 6 |
| 225 | 4 |
| 300 | 3 |
| 375 | 3 |
| 450 | 2 |
| 600 | 2 |
How to read the result
Joint dimensions matter much more than length: doubling width and depth to 12 × 12 mm makes the same 20 m joint need four times the sealant. A common rule of thumb for movement joints is a depth of about half the width for wider joints, controlled with a backer rod, and depth equal to width for narrow ones; check the sealant's data sheet. A triangular fillet bead in a corner, such as around a bath, uses roughly half the volume of a square joint of the same size, so this calculator will overestimate it. Common cartridge sizes are about 280–310 mL; foil sausages are often 400 or 600 mL.
Common mistakes
- Filling deep joints without a backer rod, which wastes sealant and restricts movement.
- Measuring joint width at the surface only when the gap tapers.
- Using cartridge nominal size when part of it cannot be extruded.
- Choosing an indoor acrylic for an outdoor movement joint.
Frequently asked questions
How many metres does a 300 mL cartridge seal?
About 8 m at 6 × 6 mm, before waste, or about 33 m for a 3 × 3 mm bead.
What is a backer rod for?
It controls joint depth, saves sealant and prevents three-sided adhesion so the sealant can stretch.
Should I seal a bath full or empty?
Many installers fill the bath with water first so the joint is sealed at its widest position.
Sources: ASTM C1193 — Standard Guide for Use of Joint Sealants