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The layers people miss

Most sellers price from grams of filament. The real cost has five parts:

• Filament in the part. The bit everyone counts.
• Filament that never became a part — purge from colour changes, supports, skirts, stringing. On a multi-colour print the purge block can weigh more than the model.
• Electricity. Small per print, real per year. Printer wattage × hours × your unit rate.
• The printer wearing out. A £700 machine over 5,000 productive hours is 14p an hour before you replace a hotend or a belt. Most people ignore this, so when the printer needs replacing it feels like a disaster instead of something they had planned for.
• The prints that failed. This is the one that changes the number most.

The failure-rate mistake

Say 8% of your prints fail. The instinct is to add 8% to the cost. That is wrong, and it under-prices you every time.

To end up with 100 sellable units at an 8% failure rate you have to start about 109. So the cost of the successes has to carry the failures — which means dividing by (1 − 0.08), not multiplying by 1.08. At 8% the difference is small. At 30% it is the difference between a 43% uplift and a 30% one, and at 50% the naive version leaves you a quarter short of your real cost — put the other way, the true figure is a third higher than the one you would have used.

The higher your failure rate, the more this matters, so the mistake costs you most when things are already going badly.

A worked example

A print that produces 4 units, taking 5 hours on a 350W machine:

• 200g of filament at £0.022/g — £4.40
• Electricity: 350W × 5h = 1.75 kWh at 32p — £0.56
• Machine: 5h at 26p/hour depreciation and wear — £1.30
• Subtotal — £6.26
• Failure uplift at 8%: £6.26 ÷ 0.92 — £6.80

£1.70 per unit, against the £1.10 you would get from filament alone. Price at a 40% margin and that is £2.83 rather than £1.83, a pound more on every unit you sell.

Margin is on price, not on cost

One more place the arithmetic bites. A 40% margin means cost is 60% of the price, so price = cost ÷ 0.6. Treating 40% as a markup on cost (cost × 1.4) gives you £2.38 instead of £2.83 and a margin of 29%, not 40%. Both numbers look like "forty percent" until you check.

What to actually do

You do not need software for this. You need to log prints as they happen — including the failures, which is the part everyone skips — and weigh things occasionally rather than trusting slicer estimates, which drift with settings and filament moisture.

Record your failures because they make the biggest difference to the figure, and nobody remembers them accurately. A month on, most people think their failure rate was lower than it really was.