Remaining Filament Calculator
You weighed your spool on a scale. How much filament is actually left? Subtract the empty spool weight, get the remaining grams, and see whether it's enough for your next print or if it's time to reorder.
Looking for one brand's numbers? Dedicated pages: Bambu Lab, Polymaker, eSun, Prusament.
How this works
The math is one subtraction. Take what your scale reads, subtract the weight of the empty spool itself, and what's left is filament. Divide by the original spool weight and you get a percent remaining.
The trick is knowing the empty spool weight. Spool weights vary surprisingly: a Bambu reusable spool with a refill core is around 233 g, a Polymaker cardboard spool is around 140 g, and most generic plastic spools are 200-230 g. We've included presets for the common brands so you don't have to weigh an empty spool every time.
The formula in detail
remaining = current_total_weight - empty_spool_weight
percent = remaining / original_filament_weight × 100
Worked example. You set a Polymaker spool on a kitchen scale and it reads 540 g. The empty Polymaker cardboard spool weighs about 140 g. So remaining = 540 - 140 = 400 g. From a 1 kg spool, that's 400 / 1000 × 100 = 40% remaining. At $22/kg, the value remaining is 0.40 × 22 = $8.80.
Common empty spool weights
| Spool | Empty weight | Notes |
|---|---|---|
| Bambu reusable spool + refill core | ~233 g | Reusable spool (~208 g) plus the refill's cardboard core (~25 g) |
| Bambu cardboard spool | ~205 g | Full cardboard spool when you don't buy refills |
| Polymaker cardboard spool | ~140 g | Current standard; older plastic spools are ~215 g |
| eSun | ~200 g | Standard plastic spool |
| Prusament | ~200 g | Cardboard-core spool, community-measured 192-207 g |
| Generic plastic spool | 200-250 g | Most other brands fall in this range |
| Heavy "premium" spool | 260-280 g | Some specialty filaments use heavier spools |
The cleanest workflow: weigh one empty spool of each brand you use, write the number on the spool with a Sharpie, never have to guess again.
When the result feels off
- Calculator says you have more than the original weight. Check all three inputs: current loaded weight, empty-spool tare and original net filament amount. The warning can result from more than one input; do not change a measured tare just to make the percentage fit.
- Calculator says zero or negative. Either the spool really is empty, or the empty spool weight is higher than the current scale reading. Worth weighing an empty spool to verify.
- Number seems too low compared to your slicer estimate. Some scale variation is normal (1-2 g). Check the scale by placing a known-weight object on it (a kilogram is a 1 L bottle of water).
Frequently asked
- Why does empty spool weight matter so much?
- If you assume the wrong empty spool weight, the calculation is off by exactly that amount in grams. A Polymaker cardboard spool is around 140 g, while a generic plastic spool can be 220 g or more. That's an 80 g swing, roughly a third of a small print. Use the right preset for your spool brand or weigh one empty spool of each type you keep around.
- How do I weigh the spool?
- Any kitchen scale that reads to the nearest gram works. Postal scales are also fine. Take the spool off the printer, set it on the scale, read the number. The spool side hub doesn't need to be removed.
- What if the calculator says I have more filament than the original spool weight?
- Check the current loaded weight, the matching empty-spool tare and the original net filament amount. A low tare, a high scale reading or an original-fill entry that is too small can each produce this result. The warning alone does not identify which input needs correcting.
- Will this work for non-1kg spools?
- Yes. Adjust 'Original filament weight' to match. Common alternatives: 250 g sample spools, 500 g half spools, 750 g spools (some Polymaker), 2 kg or 2.5 kg refill rolls.
- Can I use this to know when to reorder?
- Yes. Anything under 100 g remaining is essentially a one-print spool. If you're using that filament regularly, order before you hit 100 g so you're not waiting on shipping mid-project.