3D Print Filament: Estimating Length, Weight and Cost Before You Print

Your slicer says a print needs “8.4 m of filament” or “25 g.” Your spool is sold by weight. Your wallet cares about dollars. Connecting those three units — length, weight and cost — lets you answer the questions that actually matter: Will this spool finish the job? What did that benchy really cost me? Am I quoting a client fairly? The good news is they’re linked by simple geometry.

From length to weight

Filament is a cylinder, so its volume is the cross-sectional area times the length:

volume (mm³) = π × (diameter / 2)² × length
weight (g)   = volume (cm³) × density (g/cm³)

The two constants that matter are diameter (1.75 mm or 2.85 mm) and material density. For standard 1.75 mm filament, the cross-section is π × 0.875² ≈ 2.405 mm². So one metre (1000 mm) is about 2405 mm³ = 2.405 cm³ of plastic.

Multiply by density and you have grams per metre:

Material	Density (g/cm³)	Grams per metre (1.75 mm)
PLA	1.24	~2.98 g/m
PETG	1.27	~3.05 g/m
ABS	1.04	~2.50 g/m
TPU	1.21	~2.91 g/m
Nylon	1.14	~2.74 g/m

So a print that uses 8.4 m of PLA weighs roughly 8.4 × 2.98 ≈ 25 g — which is why those two slicer numbers always line up.

From weight to cost

Cost is the easy step once you know weight. If a 1 kg spool costs $22:

cost per gram = spool price / spool grams = 22 / 1000 = $0.022
print cost    = grams × cost per gram

That 25 g benchy costs 25 × $0.022 = $0.55 in material. The Filament Length, Weight & Cost calculator does all three conversions at once — feed it any one value (metres, grams or dollars) plus your spool price and diameter, and it fills in the rest.

Why diameter precision matters

Notice the diameter is squared in the volume formula. That means small diameter errors compound: a filament running at 1.80 mm instead of 1.75 mm carries about 6% more plastic per metre. Over a full spool that’s real money and real weight — and it’s why cheap, out-of-tolerance filament can throw off both your cost estimates and your print quality.

Resin and total job cost

If you run a resin printer instead, the math changes — resin is priced by volume (millilitres), not length. The Resin Print Volume & Cost calculator estimates material use from your model’s volume and resin price.

And material is only part of the true cost of a print. Electricity, machine wear and your time all add up. When you’re quoting a job or pricing prints for sale, the Print Time & Cost estimator folds print duration, power draw and labour into a single number.

Frequently asked questions

Will a 1 kg spool finish my 8-hour print? Check the slicer’s gram estimate against what’s left on the spool. As a rule of thumb, weigh the spool and subtract the empty-spool weight (often ~200 g) to know your remaining filament.

Does color or brand change the weight math? Density varies slightly by additive and brand, but staying with the per-material figures above gets you within a few percent — fine for planning.

Why does my print weigh more than the estimate? Infill, supports and a brim all add plastic the headline figure may not emphasise. The slicer’s total includes them; eyeballing the model doesn’t.

Price your next print in seconds with the Filament Length, Weight & Cost calculator — no spreadsheet required.