Calculate mass from a length and a matching mass-per-length specification. Animated cable segments explain why feet alone cannot convert to kilograms.
Open the feet to kg calculator · More Physics and Math videos
The calculation
Mass = length × mass per unit length; 1 ft = 0.3048 m exactly
Worked example
20 ft × 0.5 kg/ft = 10 kg. Separately, 10 ft × 0.3048 m/ft = 3.048 m; at an assumed 2 kg/m this gives 6.096 kg. A 0.75 kg/ft material would instead have a mass of 15 kg over the same 20 ft.
The mass-per-length values are hypothetical. Use the actual material and cross-section specification, and account for fittings or other components separately.
Video chapters
- 0:00 — Feet alone cannot tell you kilograms
- 0:15 — Find the mass per unit length
- 0:31 — 20 ft × 0.5 kg/ft = 10 kg
- 0:47 — Same length, different material specification
- 1:04 — Match a kg/m specification with meters
- 1:20 — 3.048 m × 2 kg/m = 6.096 kg
- 1:35 — Use the specification for the actual object
- 1:54 — Length × matching mass per length
Read the full transcript
Two cables are both twenty feet long, but one is thin and the other is thick. Do they have the same mass? Not necessarily. Feet measure length and kilograms measure mass. To connect them, we need a specification for how much mass each unit of length contains.
Suppose the actual cable specification is zero point five kilograms per foot. This is an example assumption. It says that each one-foot section of this cable contributes half a kilogram. Use a value for the correct material and cross section; a different cable can have a different value.
For a twenty-foot length, multiply twenty by zero point five. The result is ten kilograms. Visually, twenty equal one-foot sections each contribute half a kilogram. The foot in the length cancels the per-foot part of the specification, leaving kilograms as the answer unit.
Keep the length at twenty feet, but change the specification to zero point seven five kilograms per foot. The mass becomes fifteen kilograms. The length did not change; the mass per foot did. That is why there is no single feet-to-kilograms conversion factor for every object.
Now take a separate ten-foot piece with a specification of two kilograms per meter. First convert feet to meters using exactly zero point three zero four eight meters per foot. Ten feet is three point zero four eight meters. The same physical piece now has a matching length unit.
Multiply three point zero four eight meters by two kilograms per meter. The result is six point zero nine six kilograms. Check the answer by dividing the mass by two kilograms per meter: you recover three point zero four eight meters. The forward and reverse calculations agree.
A mass-per-length specification normally applies to a particular product. Fittings, connectors, coatings, or changes in cross section can need separate accounting. For a uniform solid, mass per length can also be derived from density times cross-sectional area, provided all the units match. Length by itself still is not enough.
Identify the material's mass per unit length, convert the length to match that unit, and multiply. Keep the unit cancellation visible as a check. Twenty feet at half a kilogram per foot gives ten kilograms. The missing information was the material specification, not a universal conversion between feet and kilograms.