How to measure a joint for caulk
Add up every run you mean to seal. Around a window that is the full perimeter, so a 3 by 5 foot window is 16 feet. Around a tub, it is the three sides against the wall. Trim and siding joints can run to hundreds of feet, so count them by wall instead of guessing.
Then measure how wide the gap really is at a few spots, and use the wider figure. Depth matters as much as width, and you control it. Sika's limits for Sikaflex-1A are 1/4 inch at the least and 1/2 inch at the most, with a 2 to 1 ratio of width to depth for that product. Other sealants have their own rule, so check the label.
A deep joint would swallow tube after tube if you filled it from the bottom. Sika says that where a joint or crack is deeper than 1/2 inch, you should use backer rod to control the depth. In a joint that moves, it also says to use backer rod to avoid three-point bonding. The rod runs the same length as the joint, but we could not verify a sizing rule for its diameter, so ask the supplier.
A worked example
Here is 100 feet of joint at 1/4 by 1/4 inch from 10.1 ounce tubes. These lines come from the same formula the calculator above runs.
100 ft at 1/4 by 1/4 in, 10.1 fl oz tube
- 10.1 × 1.8047 = 18.23 cu in in the tube
- 0.25 × 0.25 × 12 = 0.75 cu in per foot
- 18.23 ÷ 0.75 = 24.3 ft per tube
- 100 ÷ 24.3 × 1.1 = 4.53 tubes
- Buy 5 tubes
The 1.1 is our own 10 percent for what stays in the nozzle and what overfills, since Sika's yields assume no waste. New windows and doors bring these joints with them, so the window cost calculator and the door cost calculator are worth a look too.
How far a 10.1 fl oz tube goes
Sika's table rounds down in two places, so 12.15 prints as 12.1 and 4.05 prints as 4.0. Everything else matches this formula to a tenth of a foot.
| Width by depth | Feet per tube | Sika prints | Tubes for 100 ft |
|---|---|---|---|
| 1/4 × 1/4 in | 24.3 ft | 24.3 ft | 5 |
| 3/8 × 1/4 in | 16.2 ft | 16.2 ft | 7 |
| 3/8 × 3/8 in | 10.8 ft | 10.8 ft | 11 |
| 1/2 × 1/4 in | 12.2 ft | 12.1 ft | 10 |
| 1/2 × 3/8 in | 8.1 ft | 8.1 ft | 14 |
| 1/2 × 1/2 in | 6.1 ft | 6.1 ft | 19 |
| 3/4 × 3/8 in | 5.4 ft | 5.4 ft | 21 |
| 3/4 × 1/2 in | 4.1 ft | 4.0 ft | 28 |
| 1 × 1/2 in | 3.0 ft | 3.0 ft | 37 |
Mistakes with caulk and sealant
- Trusting the nozzle. A bead usually comes out wider than the cut, which shortens your yield. Cut small and open it up.
- Forgetting depth. A 1/2 inch wide joint gets 12.1 feet at 1/4 inch deep and 6.1 feet at 1/2 inch deep, by Sika's table.
- Filling a deep joint. It wastes sealant, and Sika's limit for this product is 1/2 inch deep. Use backer rod.
- Three-point bonding. A sealant bonded on three sides cannot move as designed. Sika says to use a backer rod to avoid it.
- The wrong product. Moving joints need a sealant rated for it, and firestop joints must follow their listed system.
Questions
How many feet does a tube of caulk cover?
About 24 feet at a 1/4 inch bead, for a 10.1 ounce tube. Sika's table prints 24.3 feet. Going to 1/2 by 1/2 inch drops that to 6.1 feet, so the size of the joint matters far more than the brand.
Does bead size matter?
Yes, a great deal, since width and depth both multiply the volume. Doubling both takes four times the sealant. A 1/4 inch bead gets 24.3 feet from a tube, on Sika's table, and a 1/2 inch bead gets 6.1 feet.
How many tubes of caulk per window?
One tube does about one and a half windows at a 1/4 inch bead. A 3 by 5 foot window has 16 feet around it, and a tube runs about 24 feet. Wider gaps use far more, so measure yours before buying.
How many tubes of caulk for a bathtub?
One tube is usually plenty at a 1/4 inch bead. A 5 by 2.5 foot tub against three walls has about 10 feet of joint, and a tube runs about 24 feet. A wider gap on an older tub can use much more.
How much backer rod do I need?
The same length as the joint it fills. Sika says to use it where a joint is deeper than 1/2 inch, to control the depth, and to avoid three-point bonding. We could not verify a sizing rule, so ask the supplier which diameter suits your gap.
What does a 300 ml tube cover?
Nearly the same as a 10.1 ounce tube. Sika labels them as equal, and by volume 300 ml is 18.31 cubic inches against 18.23. That is about 24.4 feet at a 1/4 inch bead, which is too small a gap to change what you buy.
This caulk count is an estimate from plain geometry, with only the waste you set for what stays in the nozzle or overfills. A real bead is rarely the neat shape you plan. Check the label for your sealant's joint limits, since Sika's ratio belongs to one product. Fire-rated joints follow their listed system, so this page does not size them.
Sources and review
- Sikaflex-1A product data sheet, Sika Corporation, 2025.
- Sikaflex construction sealant installation instructions, Sika, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Feet per tube
Feet per tube = Tube volume ÷ (Width × Depth × 12)
- Tube volume
- 18.23 cubic inches for a 10.1 fl oz tube
Worked through: 0.25 × 0.25 × 12 = 0.75
Tubes
Tubes = Joint length ÷ Feet per tube × Waste factor
- Waste factor
- 1 plus your waste percent
Worked through: 100 ÷ 24.3 × 1.1 = 4.53
The constants, and where they come from
TUBE_FL_OZ
Published figureCommon cartridge size, in fluid ounces
Applies to: Sika's cartridges, which many brands match
Source: Sikaflex-1A product data sheet, Sika Corporation, published 2025-10, read 2026-11.
Also from: Sikaflex construction sealant installation instructions, Sika, no date on the page, read 2026-11.
Sika's table of yields in linear feet for this cartridge matches this calculator within a tenth of a foot.
TUBE_ML
Published figureThe metric cartridge size, in milliliters
Applies to: Sika labels it as equal to 10.1 fluid ounces
Source: Sikaflex-1A product data sheet, Sika Corporation, published 2025-10, read 2026-11.
300 ml is 18.31 cubic inches and 10.1 fluid ounces is 18.23, so the two are near but not the same, and each is worked out on its own.
SAUSAGE_FL_OZ
Published figureA sausage pack, in fluid ounces
Applies to: Sika's sausage packs, which need a matching gun
Source: Sikaflex-1A product data sheet, Sika Corporation, published 2025-10, read 2026-11.
Sika lists a 20 fl. oz. (600 ml) sausage. The yield here is scaled by volume, and Sika prints a table only for the cartridge.
CU_IN_PER_FL_OZ
DefinitionalCubic inches in a US fluid ounce
Applies to: Definitional, for US fluid ounces
True by definition, so there is nothing to cite. The derivation is below.
A US gallon is defined as 231 cubic inches, and it holds 128 fluid ounces, so one fluid ounce is 231 divided by 128.
CU_CM_PER_CU_IN
DefinitionalCubic centimeters in a cubic inch
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
An inch is exactly 2.54 centimeters, so a cubic inch is 2.54 cubed. One milliliter is one cubic centimeter.
INCHES_PER_FOOT
DefinitionalInches in one foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches make a foot by definition, so width times depth times 12 is the volume in each foot of joint.
DEFAULT_WASTE_PERCENT
ModelledExtra added for nozzle loss and overfill, in percent
Applies to: Our own assumption, which you can change
No published figure exists for this. It is derived, and the derivation is below rather than a citation to something that does not say it. Last checked in November 2026.
No sealant maker we read gives a figure. Sika's yields assume no waste at all, so we add a modest allowance and let you set your own.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
- Modelled:
- derived rather than measured, and the derivation is shown.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the caulk calculator
- 1
Add up the length of every joint
- 2
Measure how wide each joint is
- 3
Choose how deep the sealant goes
- 4
Pick your tube size
- 5
Say if the joint needs backer rod
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- The bead is a square or rectangle in section, which is how Sika's table works
- Each tube's volume comes from its labeled size
- A 10 percent waste allowance is our own assumption, and you can change it
- Backer rod runs the full length of the joint
Pro tips
- Cut the nozzle small and open it up if you need to
- Check the label for the joint limits of your sealant
- Use a sealant rated for movement on joints that move