How the Filament Cost Calculator Works
Your slicer already tells you how much filament a print uses, in grams, metres or both. The calculator turns that into money from what you paid for the spool, then adds the costs people forget: the electricity, the prints that fail, and anything else that goes into the finished part.
If you only have metres, it converts them to grams with the filament's diameter and density. A metre of 1.75 mm PLA is a cylinder 1000 mm long with a 2.405 mm² cross-section, which is 2.405 cm³, and at 1.24 g/cm³ that weighs about 2.98 g. So a 1 kg spool of 1.75 mm PLA holds roughly 335 metres.
The Formulas
1. Filament cost
price per g = spool price ÷ spool g
filament cost = grams × price per g
Spool weight is the net filament weight on the label, without the empty reel.
2. Metres to grams
area (mm²) = π × d² ÷ 4
grams = metres × area × density
With metres, area in mm² and density in g/cm³ the units cancel to grams (1 m × 1 mm² = 1 cm³).
3. Electricity
kWh = average watts × hours ÷ 1000
electricity = kWh × price per kWh
Use the average draw while printing. The heaters cycle on and off, so the average is well below the power supply's rating.
4. Failures and parts per plate
per good print = (filament + power + other) ÷ (1 − failure rate)
per part = per good print ÷ parts on the plate
If 1 print in 10 fails, every 9 good prints carry the cost of 10. This counts a failed print as a whole print wasted, which errs on the safe side.
Worked Example
A plate of 4 PLA brackets: 50 g, 3 hours, an example £20 spool of 1 kg, 120 W average, an example rate of 25p per kWh, 10 % failures
- Price per gram: £20 ÷ 1000 g = £0.02, so the filament is 50 × 0.02 = £1.00.
- Length: 50 g ÷ 1.24 g/cm³ = 40.3 cm³, ÷ 2.405 mm² = 16.8 m of 1.75 mm filament.
- Electricity: 120 W × 3 h = 360 Wh = 0.36 kWh, × £0.25 = £0.09.
- Before failures: £1.00 + £0.09 = £1.09. With 10 % failures: £1.09 ÷ 0.9 = £1.21 per good plate.
- Per bracket: £1.21 ÷ 4 = £0.30. The spool makes 20 plates like this, or 80 brackets.
Practical Tips
- Trust the slicer's grams. PrusaSlicer, Cura, OrcaSlicer and Bambu Studio all show the filament weight once you slice. Check the density in the slicer's filament profile matches your material, or the grams will be off.
- Count supports, brims and purge. The slicer figure usually includes them, but a purge tower or a multi-colour print can use more plastic for waste than for the part.
- Weigh the spool. To see how much is left, weigh the spool and subtract the empty reel's weight (often printed on the label or the maker's website). The STL print estimator gives a rough gram figure for a model before you slice it.
- Measure the power once. A plug-in energy meter over a full print gives kWh directly; divide by the hours for the average watts. Reuse that figure for similar prints. For workshop machines too, see the machine electricity cost calculator.
- Selling prints? Material and power are only part of the price. Add your time, wear parts like nozzles and build plates, and your margin: the production quote calculator and the hourly rate calculator help with that.
- Save plastic where it does not matter. More walls and less infill is often stronger for the same grams; the print strength vs infill estimator compares settings.
Frequently Asked Questions
Divide the spool price by its weight in grams and multiply by the grams the slicer says the print uses. A 50 g print from a £20, 1 kg spool costs £1.00 in plastic. Add electricity and an allowance for failed prints for the full cost.
About 335 m for 1.75 mm PLA at 1.24 g/cm³, and about 126 m for 2.85 mm PLA. Lighter plastics give more length per kilogram: 1.75 mm ABS at 1.04 g/cm³ is about 400 m per kg. The table on this page lists the common materials.
It depends mostly on the heated bed temperature, the room and whether the printer is enclosed. Multiply the average watts by the hours and divide by 1000 for kWh. The only reliable figure for your printer is a plug-in energy meter reading over a whole print; the power supply's rating is the maximum, not the average.
Divide the cost of one print by (1 minus the failure rate). At a 10 % failure rate a £1.09 print becomes £1.21, because every nine good prints also pay for one that failed. Prints that fail on the first layer waste far less, so this is a cautious figure.
The conversion depends on density and diameter. If the slicer's filament profile uses a different density from the one picked here (for example a generic 1.24 for a PETG at 1.27), the grams for the same metres will differ by that ratio. Set both to your spool's data sheet figure.
Densities are typical published figures, not data for your spool. Prices in the example are illustrative; use what you actually paid and your own electricity tariff.