A UK printer owner has four costs, and plastic is the smallest.

A 1 kg spool of PLA from the familiar budget brands lands at roughly £14 to £20 delivered, which is 1.4p to 2p a gram. The engineering materials and the premium brands run from about £22 to £35 a kilo, and PETG sits between the two.

Buy on a cost-per-gram basis rather than per spool, watch for the 750 g spools sold at a 1 kg price, and remember that an unusable spool is not free: wet filament that strings and cracks costs you its price plus the prints it ruined. Keep it dry and it keeps its value.

A 70 g print is about £1.00 to £1.40 of plastic. If that number is the one deciding your price, your price is wrong.

People either ignore power entirely or imagine it is crippling. Neither is right. A 0.4 nozzle bedslinger with a heated bed averages somewhere around 100 to 150 W over a long print once the bed has come up to temperature — more with a chamber, less with a small bed and a low bed temperature.

At Ofgem's capped rate for October to December 2026, 26.32p a unit, five hours of printing is roughly 16p. A thousand hours a year, which is a machine running most evenings, is something like £30 to £40. Worth knowing, not worth losing sleep over, and worth measuring with a £10 plug meter if you want your own figure rather than ours.

The exception is a machine that prints all day in a cold room: heated beds fight the room, so a cheap enclosure pays for itself twice, in power and in warping.

A large letter, which Royal Mail defines as anything up to 353 × 250 × 25 mm and 750 g, is £1.55 second class up to 100 g and £1.90 up to 250 g with a stamp, at the prices from April 2026, and it goes in any postbox. Anything that does not fit a large letter goes as a parcel, and where you buy the label matters more than people expect. Booked online through a courier and dropped at a parcel shop, a small parcel is roughly £2 to £3. The same parcel handed over the counter as Royal Mail second class starts around £4.40. Packing adds 20p to 80p on top. On a £10 item that is a quarter to a half of the sale, before you have touched your filament.

• Design for the box. A print that fits a large letter costs a fraction of a parcel. Hinges, flat packs and parts that nest change your economics more than any slicer setting.
• Quote postage honestly or build it in. “Free delivery” simply means you pay it. Either is fine, as long as you have counted it.
• Count the trip. A drop-off is twenty minutes of your life per batch. Batch your posting, or get collections.

If you sell your own prints, somebody takes a cut before you do. Rates change, so treat these as the shape of it rather than gospel, and check the current schedule when you price.

The pattern is consistent: the easier it is to be found, the more of the sale you give away. Which is the honest argument for the routes where you are paid for printing and somebody else carries the selling — see print on demand with your own printer.

HMRC has a trading allowance for casual income from a side activity. At the time of writing it is £1,000 in a tax year: under it, you do not need to register or report that income; over it, you register as self-employed and file a return, deducting your real costs — filament, postage, the printer, power, packaging.

We are not accountants and this is not advice. Rules and thresholds change; check the current position on GOV.UK before you rely on any of it.

The practical lesson is to keep receipts from the first sale, not from the month you cross a threshold. A folder of spool invoices and a postage spreadsheet costs you nothing now and saves an evening later.