Filament Weight to Length Calculator
Find out how much printable filament a weight represents. Weigh what's left on a spool, subtract the empty reel, and convert the remaining grams into metres so you know if a print will finish.
Density varies by brand and colour — check your spool's spec sheet and adjust if you need an exact figure.
Remaining filament only. Weigh the whole spool, then subtract the empty-spool (tare) weight printed on the reel.
Formula & how it works
Cross-sectional area is π × (diameter ÷ 2)² in square millimetres. One square-millimetre of strand a metre long is exactly one cubic centimetre, so grams per metre is area × density.
Length is then weight ÷ (area × density), giving metres from grams.
Worked example
A 1.75 mm PLA strand has a cross-section of about 2.41 mm², so at 1.24 g/cm³ it weighs roughly 2.98 grams per metre. A full 1,000-gram spool therefore holds about 335 metres. A 250-gram leftover works out to roughly 84 metres — around 275 feet — plenty for several small prints.
Making it accurate
Subtract the empty spool first
The number that matters is the filament weight, not the total on the scale. Cardboard reels are light, but plastic ones can be 200 grams or more, so include the tare printed on the spool or the maker's spec. Skipping this is the most common reason a "how much is left" estimate comes out too high.
Diameter drives the length
Thicker filament packs more plastic into each metre, so it runs out faster by length. A kilogram of 2.85 mm filament is only about 125 metres against roughly 335 for 1.75 mm. Set the diameter to your actual filament before trusting the result.
Leave yourself a margin
Because diameter and density wander slightly along a spool, treat the length as an estimate and keep a little in reserve. If a slicer says a print needs close to what you have left, load a fresh spool rather than risk running dry partway through.