How Much Filament Do I Need?

Two questions come up constantly at the bench: will this print finish on what’s left, and how much filament does this project actually need. Both come down to the same conversion between weight and length, which depends only on the material’s density and the filament’s diameter.

How Much Is On a Full Spool

ABS gives you noticeably more length per kilogram than PETG for the same money, because it’s the least dense of the common materials. That matters for cost per part, not just per kilo.

The Conversion

Filament is a cylinder, so the maths is short:

length (m) = mass (g) / (density (g/cm³) x π x radius (cm)² ) / 100

For 1.75mm filament the cross-section is about 0.0241 cm²; for 2.85mm it’s about 0.0638 cm². So a metre of 1.75mm PLA weighs roughly 3 grams, and a metre of 2.85mm PLA weighs roughly 8 grams.

Going the other way is the same equation rearranged, which is what the filament calculator does instantly for any material and diameter.

What the Slicer Tells You

Every slicer reports both a weight and a length estimate after slicing, usually in the summary panel next to the print time. Use the weight figure, because that’s the number you can check against a scale.

Slicer estimates are good but not exact. They assume perfect extrusion and don’t know about your flow multiplier, purge lines, or a failed first attempt. Budget about 5% over the estimate, and more if you’re running supports or a purge tower for multi-colour.

How Much Is Left On This Spool

You can’t see it and the manufacturer doesn’t help you, so weigh it.

remaining filament = total spool weight on the scale - empty spool weight

The empty spool weight is the part people don’t have. Options, best first:

1. Check the manufacturer’s spec. Many brands print the empty spool weight on the label or list it on their site. Values typically run 130-250g depending on material.
2. Weigh an empty spool you’ve already finished from the same brand. They’re consistent within a product line.
3. Estimate by type: cardboard cores tend to run 130-180g, injection-moulded plastic 180-250g, and reusable snap-together spools around 200-220g.

Get this number once per brand and you never think about it again. It’s exactly why an inventory that stores the tare weight beats doing arithmetic at the bench each time: Gyroid subtracts it for you, so weighing a spool updates grams remaining in one step.

Rules of Thumb

Handy approximations for 1.75mm at 0.2mm layers and 15-20% infill:

• A 3DBenchy - about 12 g, roughly 4 m.
• A 20mm calibration cube - about 4 g.
• A phone stand - 20-40 g.
• A typical desk organiser or small box - 60-120 g.
• A full-face cosplay helmet, thin-shelled - 400-800 g, usually across multiple pieces.
• A large storage bin, 200mm cube - 300-500 g.

A 1kg spool of PLA is therefore about 80 Benchies, or two to three helmet-scale projects, or a very large number of calibration cubes you will regret printing.

Budgeting a Project

For anything multi-part, work in weight, add contingency, then convert once at the end.

1. Slice every part and note the weight estimates.
2. Add them up.
3. Add 10% for purge, skirts, and the odd reprint. Add 20% if any part is tall, warp-prone, or unsupported.
4. Compare against what you have, not against a full spool.

Point four is where projects go wrong. Two half-used spools of the same colour are not interchangeable with one full spool if there’s a dye-lot difference, and a part that changes colour mid-print because you ran a spool out is a reprint, not a save.

Don’t Run a Spool Out Mid-Print

The practical failure isn’t running out of filament in the abstract, it’s running out at hour nine of a twelve-hour print. Two habits prevent it:

• Check remaining against the estimate before you start, not after.
• Set a low-filament threshold so a spool flags itself before it becomes the reason a print failed.

Gyroid deducts filament from the spool when you log a print and flags spools below your threshold, so the arithmetic happens once and the warning arrives before the print does. Run the numbers first with the filament calculator or the cost calculator if you want the price alongside the grams.