A filament spool is a cartridge wearing a different costume: a fixed mass of consumable at a fixed price, and a part cost that’s just division. The math is friendlier than printer-ink math — no coverage percentages, no yields measured in lab conditions — but it has its own three traps: grams vs meters, the spool’s net weight, and the waste nobody counts. Here’s the honest version.

The base formula

cost per part = part grams × (spool price ÷ spool net grams)

At the canonical $20/kg commodity PLA that’s $0.02/gram — a 25 g calibration cube is fifty cents, a 150 g bracket is $3. Everything else in filament economics is a refinement of this one division:

• Premium materials move the multiplier, not the method. $45/kg PETG-CF → 4.5¢/g; $90/kg nylon-CF → 9¢/g; $120/kg PEEK-adjacent engineering filament → 12¢/g. A 100 g engineering part is $9–12 in material — still cheap against machining, no longer free.
• Cost per meter exists only for exotic filament. Most filament prices by weight; where meter-pricing appears, the conversion below handles it.

Meters ↔ grams: the density table

Slicers report both grams and meters; spools sell by grams. The conversion is pure geometry — cross-section area × length × density:

grams per meter = π × (diameter/2)² × 100 × density   (diameter in cm, density in g/cm³)

material	density	1.75 mm g/m	2.85 mm g/m
PLA	1.24 g/cm³	~2.98	~7.91
ABS / ASA	1.05	~2.53	~6.70
PETG	1.27	~3.05	~8.10
TPU	~1.21	~2.91	~7.72
Nylon (PA)	~1.14	~2.74	~7.27

Rule of thumb for the common case: 1.75 mm filament ≈ 3 g/m, so a 1 kg spool ≈ 330 m (PLA ~335, ABS ~395, PETG ~328). A slicer estimate of “12.4 m” is ~37 g of PLA — $0.74 at $20/kg. The calculator’s filament block has this conversion built in.

The waste factor everyone forgets

Your slicer’s gram estimate is what lands in the part — not what leaves the spool. Real prints spend material on:

• Skirt/brim/raft and priming — ~0.5–3 g per print, fixed overhead that punishes small parts proportionally more.
• Supports — 5–30% of part weight on complex geometry, all of it destined for the bin.
• Failed prints — the honest number. A printer running reliably still fails a few percent of jobs; a flaky setup fails 20%+. Failures cost the full partially-printed mass.

The convention that keeps estimates honest: multiply slicer grams by 1.10–1.20 (the calculator’s waste field defaults to 15%). A $0.50 benchy becomes $0.58 — unimportant. A production run of 200 parts at 80 g becomes $18.40 of extra spend — a line item worth having.

Net weight and the spool-end problem

Two spool subtleties:

1. Net vs gross. “1 kg spool” should mean 1 kg of filament; some listings ship 1 kg including the ~120–250 g spool itself. If the weight looks off — a “1 kg” spool that dies early — weigh it: an empty spool tare tells you what you actually bought.
2. The remainder is dead stock for long prints. You can’t start a 200 g part on 40 g of leftovers. Spool ends are real material but unusable for anything that won’t fit — the honest CPP of your last 10% of every spool is effectively infinite unless you queue small parts for it. Practical rule: check remaining grams (spool weight − empty tare) before any print over ~150 g.

Where electricity lands (spoiler: small)

FDM printers draw ~50–300 W once hot — bed and nozzle heating dominate, motion is trivial. At $0.16/kWh:

• 4-hour PLA print (~120 W avg): ~0.5 kWh ≈ $0.08
• 24-hour ABS print in a heated chamber (~250 W avg): ~6 kWh ≈ $0.96

Electricity is a rounding error on PLA and a real-but-small line on heated engineering materials. The genuinely expensive inputs are time and failures — material and power are both cheap, which is precisely why the waste factor matters more than either.

The whole thing in one example

A 12-hour PETG enclosure, slicer says 186 g, $28/kg spool, 15% waste, ~200 W average:

• material: 186 g × $0.028 × 1.15 = $5.99
• electricity: ~2.4 kWh × $0.16 = $0.38
• total: ≈$6.37 — of which the filament waste margin ($0.78) is bigger than the power bill.

That’s the honest shape of 3D-print economics: parts are cheap, waste is the stealth line, and time is free only because nobody invoices it. Run your own spool and part in the calculator’s filament block.

Frequently asked questions

How much does a typical 3D print cost in material?

At $20/kg PLA, material is $0.02/gram — a 25 g part is ~$0.50, a 100 g functional part ~$2, a 400 g helmet-scale print ~$8. Add 10–20% for waste/failures and it stays cheap: filament material cost is usually the smallest reason 3D prints feel expensive (time is the big one).

My slicer says 8.2 meters — how do I get grams?

Multiply by grams-per-meter for your diameter and material: for 1.75 mm PLA ≈3.0 g/m, so 8.2 m ≈ 24.5 g. The full table is above; 2.85 mm is ≈7.9 g/m for the same material. The calculator's filament block does this conversion directly.

Is '1 kg spool' really 1 kg of filament?

Check whether the listing means net filament weight or gross (spool + filament together — the plastic spool itself is ~120–250 g). Reputable listings say '1 kg net filament'; cheap ones can quietly mean gross. The calculator takes net weight — if you only know gross, subtract ~200 g for a typical spool.

What does electricity add per print?

A typical FDM printer draws ~50–300 W while running — at $0.16/kWh that's ~$0.01–0.05/hour. A 12-hour print adds maybe $0.30–0.60 — real but small next to material on most jobs. Heated-chamber machines (ABS-class materials) sit at the top of that band.

Why did my spool 'run out' with filament still on it?

The tail end of a spool is unusable for long prints — you can't start a 200 g job on 40 g of remainder. Spool leftovers are effectively dead stock unless you plan small parts for them. Some makers weigh the spool before a print to check feasibility: remaining grams = current spool weight − empty spool weight.