The bead is a volume, not a length
Coverage charts on the back of a cartridge give you feet per tube at a few bead sizes, and they are correct, but they hide the shape of the relationship. Sealant fills a cross-section, and the cross-section of a square joint goes up with the square of the dimension. A quarter inch square joint is 0.0625 square inches. A half inch square joint is 0.25 square inches — four times as much, from a bead that is only twice as wide.
Worked from first principles: a standard 10.1 fluid ounce cartridge holds about 18.2 cubic inches, since a US fluid ounce is 1.805 cubic inches. Divide by 0.0625 square inches and you get 291 inches, which is a little over 24 feet of quarter inch joint. At half an inch the same tube reaches barely six feet. That factor of four is the reason a job that took two tubes last time takes eight this time, and nobody notices until the second trip to the store.
Three bead shapes and which one you have
| Shape | Cross-section | Where you get it |
|---|---|---|
| Filled joint | width x depth | A gap between two materials that you fill flush — a control joint, an expansion gap, a gap between trim and wall |
| Round bead | pi x diameter squared / 4 | A bead laid on a surface with nothing to sit into, such as a lap joint or a flashing seam |
| Triangular fillet | width x depth / 2 | The inside corner between two surfaces, tooled to a concave face — tub to tile, countertop to backsplash |
Most interior caulking is the triangular fillet, and it uses roughly half the material of a filled joint of the same nominal size, which is why generic coverage figures overstate the tube count for trim work and understate it for structural joints. Pick the shape that matches what you are actually doing rather than defaulting to one number.
Depth, backer rod, and three-sided adhesion
For anything that has to move — an expansion joint, a joint between two different materials that expand at different rates, an exterior joint that sees temperature swings — the depth of the sealant matters as much as the width. A sealant bonded to the two faces of the joint can stretch. A sealant that has also bonded to the back of the joint is anchored on three sides and cannot, so it tears instead. Backer rod is closed cell foam pushed into the back of the joint to prevent that bond and to control the depth at the same time.
The working rule for an elastomeric sealant in a moving joint is a depth of roughly half the width, with a minimum around an eighth of an inch and a maximum around half. Rod diameter runs about 25 percent larger than the joint so it compresses in and stays where you put it. For non-moving interior trim caulk none of this applies and you are just filling a gap.
Waste, and the tube you throw away half full
Fifteen percent is the default here and it is not padding. The first inch out of any tube is air and unmixed material. The nozzle sets solid between sessions and comes out as a plug that costs you another inch. Tooling a bead pushes material out at both sides that you wipe off and bin. And a partly used cartridge left for a fortnight is usually a solid cartridge, which is why buying one large tube for a job you will do over three weekends is often false economy against three small ones.
If the count comes out above about a dozen cartridges, the economics shift to sausage packs and a barrel gun. The material is cheaper per ounce, there is less packaging waste, and reloading is faster than the cartridge swap, which matters when you are trying to run a long joint without a stop in it.
Where this fits with the rest of the work
Caulk goes on near the end, over surfaces that other calculators size. The trim calculator gives the linear feet of baseboard and casing that will need a bead along the top and against the wall, and the tile layout calculator settles the corners and changes of plane in a tiled room, which are exactly the joints that get sealant rather than grout. Grout belongs on flat surfaces between tiles and sealant belongs at every corner and every change of plane, because those are the places the assembly moves.
Questions people ask
How many feet does one tube of caulk cover?
It depends entirely on the bead size, and the relationship is not linear. A standard 10.1 fluid ounce cartridge holds about 18.2 cubic inches. At a quarter inch square joint that is roughly 24 feet, at three sixteenths about 43 feet, at an eighth about 97 feet, and at half an inch about six. For a triangular fillet in a corner, which is what most interior trim caulking actually is, halve the cross-section and double those figures. The single most useful thing to know is that going up one bead size costs far more material than it looks like it should, because you are changing an area, not a length.
What is backer rod for and do I need it?
It does two jobs. It sets the depth of the sealant so you are not filling a deep joint with expensive material that does nothing, and it stops the sealant bonding to the back of the joint. That second job is the important one in anything that moves. Sealant bonded on two faces can stretch as the joint opens; sealant also bonded to the back is held on three sides and tears rather than stretching. For a moving joint, an expansion joint, or any exterior joint wider than about a quarter inch, use rod. For interior trim caulk filling a static gap between a baseboard and a wall, you do not need it.
Should the joint be as deep as it is wide?
For a moving joint, no. The guidance for elastomeric sealants is a depth of about half the width, within a range of roughly one eighth to one half of an inch, which keeps the bead flexible enough to accommodate movement without tearing. A deep, narrow bead is stiff and fails at the edges. Static interior joints are more forgiving and are usually just filled, since nothing is asking them to stretch. If you have a wide joint and no rod, the temptation is to fill the whole depth with sealant; resist it, because you are spending three times the material to make a joint that performs worse.
Why does my waste come out so high?
Because caulk waste is mostly fixed per tube rather than proportional to the job. Every cartridge loses the first inch to air and unmixed material and another inch to whatever set in the nozzle since last time. On a job that uses two tubes that is a large fraction; on a job that uses twenty it is not. The fifteen percent default is a reasonable middle. If you are running one long continuous joint in a single session with a fresh tube, drop it to five. If you are doing a bathroom over three weekends with the same tube, budget on losing the tube between sessions and count each session separately.
Can I use this for adhesive or grout instead?
For anything sold in the same cartridge and applied as a bead, yes, the volume arithmetic is identical — construction adhesive, subfloor adhesive, panel adhesive and gutter sealant all work out the same way, and you only need the tube volume and the bead cross-section. Grout is different and this will not help, because grout fills the space between tiles across a whole floor rather than running a line, and its quantity depends on the tile size, the joint width and the tile thickness together. That belongs in the tile calculator. Thinset is different again and is sold by coverage per bag at a given trowel notch.