Log Weight Calculator

Background.

Before you lift, winch or haul a log you want some idea of its weight, and the straightforward way to get one is to treat the log as a cylinder and multiply its volume by the density of the wood. Volume needs only two tape measurements, diameter and length. Density is the hard part, because the same species can weigh very differently as a log cut this morning and as one that has dried for a season.

Diameter does most of the work. The cross-section grows with the square of the diameter, so a log 10% fatter, 33 cm instead of 30 cm, is 21% heavier, while 10% more length adds only 10%. A 30 cm log 4 m long, in wood of 600 kg/m³, comes out near 170 kg; double the diameter to 60 cm and the same length weighs about 679 kg, four times as much.

The density is yours to bring. The Wood Handbook published by the USDA Forest Products Laboratory tabulates specific gravity for a long list of species, both green and at 12% moisture content, and any water held in the wood counts toward the mass. Fresh sapwood can hold a great deal of it, so a green log may weigh far more than the seasoned timber it will become. Pick a density that matches the condition of the log in front of you, not a figure meant for kiln-dried boards.

The shape is idealised too. Real logs taper from butt to top, carry bark, and can hide rot, hollows or heavy knots. Treat the answer as a planning figure and leave a margin before choosing a strap, a trailer or a crane.

What is log weight calculator?

Log weight here is the mass of an idealised log: a solid cylinder whose diameter is the log's mean diameter, whose length is the measured length, and whose every cubic metre has the density you enter. The answer is in kilograms, the same figure a scale or a crane load cell would read. Because the density is supplied rather than assumed, the same page can describe a green log, an air-dried one or a seasoned one; the result is only as right for your log as that density is.

How to use this calculator.

1. 01Enter the mean diameter in metres, not centimetres: 30 cm is 0.3. On a tapered log, measure at the middle of its length or average the two end diameters, and decide whether you are measuring over or under the bark.
2. 02Enter the length in metres from one cut end to the other. Branch stubs and root flare have no place in the cylinder, so leave them out of the measurement.
3. 03Enter a bulk density in kg/m³ that matches the log's moisture state. If a wood table gives you a specific gravity instead, multiply it by 1,000 and by (1 + moisture content ÷ 100), using a specific gravity defined for that same moisture condition.
4. 04Read the idealised mass, then the model-scope note beside it, which lists what the cylinder leaves out. Add a margin for taper, bark and hidden defects before you size lifting gear or a load.

The formula.

m = ρ × π × d² × L ÷ 4. The term π × d² ÷ 4 is the area of a circle of diameter d, in square metres; multiplying it by the length L in metres gives the volume of the cylinder in cubic metres, and multiplying that by the density ρ in kilograms per cubic metre gives the mass in kilograms. Apart from π there are no built-in constants, and nothing is added or taken away for bark, taper or moisture: all of that has to be carried by the numbers you enter. Mass rises in direct proportion to density and to length but with the square of the diameter, so a small percentage error in the diameter shows up roughly twice over in the result. Using a mean diameter on a tapered log slightly understates the volume. A 4 m log with 25 cm and 35 cm ends is a truncated cone of about 0.2854 m³, against 0.2827 m³ for a 30 cm cylinder, a difference of about 1%.

A worked example.

Frequently asked questions.

References& sources.

1. [1]USDA Forest Products Laboratory, Wood Handbook, FPL-GTR-282.
2. [2]BIPM SI Brochure.
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.