Most caulk estimates are rules of thumb passed around until nobody knows where they came from. This one is derived: a standard cartridge holds 10.1 fluid ounces, which is 18.2 cubic inches, and a bead is a cylinder of caulk lying on its side.
The arithmetic
A bead of diameter d has a cross-sectional area of pi times d squared over four. A quarter inch bead is 0.0491 square inches in section.
Divide the cartridge volume by that and you get the length of bead one tube produces: 18.2 divided by 0.0491 is 371 inches, or about 31 linear feet.
That matches the coverage charts manufacturers publish, which is a good sign — the geometry and the industry figures agree. Where they differ is that a chart gives you four numbers and this gives you any bead size you like.
Why bead size dominates
Because the area goes with the square of the diameter, doubling the bead quadruples the consumption. A tube that runs 123 feet at a 1/8 inch bead runs only 31 feet at 1/4 inch.
This is the single reason caulk estimates come out wrong. People picture the length of the joint and not the size of the bead, and the bead is the term that actually moves.
It is also why sizing the nozzle cut matters. Cutting the tip further back than you need is a decision to use two or three times as much material.
Matching bead to joint
A tight trim joint — where baseboard meets a wall, or at a mitre — wants a small bead, an eighth of an inch or less. Anything larger looks like a repair.
A typical interior joint around a window casing or a tub surround takes three sixteenths to a quarter inch.
Exterior joints in siding, around windows and along a sill take a quarter inch or more, because they have to absorb seasonal movement. A joint that moves needs enough material in it to stretch.
Joints wider than about half an inch should not be filled with caulk alone. Backer rod goes in first — a foam cord that fills the depth and gives the caulk a correct hourglass profile so it can stretch without tearing.
Waste is real here
Ten percent is the default and it is conservative. Caulk that sets in the nozzle between sessions is gone, a tube left open overnight often skins over, and the first inch out of any tube is inconsistent.
Buy one more tube than the arithmetic says. A half-used tube costs little; stopping mid-run to go to the store while the first half skins over costs the joint.
The right product matters more than the amount
Interior trim wants a paintable acrylic latex with silicone. Wet areas — tubs, showers, sinks — want a pure silicone, which is not paintable but does not grow mould the way latex does.
Exterior joints that move want a polyurethane or a high-grade elastomeric, not a general purpose latex. The cheap tube is the expensive choice on anything exposed to weather, because you will redo it.