How long does a 1 kg filament spool last

A 1 kg spool of 1.75 mm PLA holds about 335 metres of filament. In 2.85 mm the same kilo comes to about 126 metres, because the strand is much thicker. And in ABS, which weighs less per cubic centimetre, the kilo reaches about 400 metres. The full table:

Where those metres come from

No mystery: filament is a cylinder. You divide the weight by the density to find how many cubic centimetres of plastic there are, and divide that volume by the cross-section of the strand. A 1.75 mm filament has 0.0241 cm² of section; a 2.85 mm one has 0.0638 cm², that is 2.65 times more. You can run the conversion with your own numbers in the filament grams to metres converter.

That formula also explains the 2.85 mm column, which looks alarming the first time you see it: the same kilo comes to 126 metres, 38% of what it gives in 1.75.

It is not that it yields less. It is the same thousand grams and the same part: what changes is that every metre of the fat strand carries 2.65 times more plastic. The 60 gram planter eats 20.1 metres in 1.75 mm and only 7.6 metres in 2.85. If you compare spools by the metres they advertise without looking at the diameter, you reach the opposite conclusion.

Why the metres never come out exact

PLA's 335 metres assume a strand of exactly 1.75 mm, and no filament is exact. Serious brands declare a tolerance of ± 0.02 mm, and that tiny variation shows up in the total:

Between the two extremes there are fifteen metres of difference on the same one-kilo spool. That is why any figure in metres is approximate, and why grams are always more reliable than metres: weight does not depend on how fat the strand turned out.

And one practical consequence that is not about accounting: if your spool comes out consistently thick or thin, measure the real diameter with callipers at several points and type it into the slicer profile. With the right value, the flow corrects itself.

How many parts come out of a spool

Careful with these figures: they are the theoretical maximum. In practice you have to knock off the filament stranded on the core — the last 3 to 5 metres can't be printed safely — the purges when you change colour, and the failed prints. Counting on 90% usable is realistic.

From the theoretical kilo to the real one

That 90% deserves an explanation, because what people think they lose and what they actually lose are not the same thing.

The five metres left on the core sound like a lot and they are 15 grams: 1.5% of the spool. If it is three metres, nine grams. In other words, the end of the reel barely counts.

What takes the rest is failed prints and, if you change colour often, the purges. That is why the 10% worth knocking off is not a loss in the material: it is a reflection of your failure rate, and there you can actually do something, as why 3D prints fail sets out.

With that 90%, the $20 spool stops costing $20 per usable kilo and starts costing $22.22, 11% more. And the units per spool drop like this:

On small parts the difference is twenty units; on big ones, a single one. But that single one can be exactly the one you are short of to finish an order.

How long it lasts in time, not metres

Another way to measure it: a 1 kg spool covers about 80 hours of printing at a normal rate of roughly 12 grams an hour. Translated into calendar days:

Those 80 hours cost power and machine too: about $1.44 of electricity at 120 W and $0.15/kWh, and around $6 of depreciation on a $300 printer. Put another way, getting through a whole spool costs around $7.50 on top of the price of the plastic, and that is what hardly anybody adds up. The full breakdown of that hour is in what an hour of 3D printer time costs.

How many spools you get through in a year

It works the other way round too, and that is the sum you need to know what you will spend on filament: starting from the parts you print in a year, with the 60 gram reference part.

And for one specific order the sum fits on a line: parts × grams ÷ 900 — nine hundred, not a thousand, because the 10% is already knocked off — and round up. Fifty 60 gram planters are 3.3 spools, so you buy four.

Buying exactly enough is the classic mistake: being half a part short of the last unit with the client waiting costs far more than one spare spool in the cupboard.

How to know what's left on an open spool

1. Weigh the whole spool on a kitchen scale.
2. Subtract the weight of the empty core: between 200 and 250 g on plastic ones, less on cardboard. Plenty of brands print it on the spool itself.
3. What's left is the grams of filament; convert them to metres with the converter.

And before you start a long print, check you'll make it: if you have 40 metres of 1.75 mm PLA left, that is about 120 grams, and a part asking for 130 g will stop halfway through the night.

If your core has no weight printed on it, weigh it once when it runs out and write it down: spools from the same brand all weigh the same, and with that number the subtraction works forever.

What to do with the end of the reel

The last metres come off tightly curled, pull on the extruder and cause trouble on long parts. Throwing them out is an option, but it is fifteen grams and there are better uses.

• Calibration cubes. Each one is 10 grams and it is exactly what you want printed when you open a new spool: it tells you whether that brand extrudes like the last one.
• Test parts. The first attempt at a new part does not have to come out of fresh filament, and there it hardly matters that the strand is curled.
• Temperature towers. The classic nobody prints out of laziness, and which is made precisely from leftovers.

What is not worth doing is splicing it onto the next spool. The join passes through the extruder and the nozzle, and if it opens up ten hours into a print you have lost the part and the ten hours.

What really finishes a spool off

It is not using it up: it is moisture. Filament absorbs water from the air, and a damp spool prints badly even when it is full.

The symptoms are recognisable: crackling as it extrudes, bubbles, stringing everywhere and a rough surface where it used to come out smooth. The part is not just ugly, it is weaker.

Each material puts up with its own amount. PLA is forgiving and lasts months in a sealed bag with desiccant. PETG asks for a bit more care. Nylon and TPU go off in days in the open air, and with those a filament dryer stops being a luxury.

It is worth seeing in money: a $20 spool ruined by leaving it open is the equivalent of fifteen planters from the table that you are not going to print.

Are the big spools worth it?

The 3 and 5 kg formats work out cheaper per kilo, and that part is not up for debate. What is worth looking at first is the time.

Five kilos are 400 hours of printing in the same colour. At three hours a day that is over four months without changing spool, and over that stretch moisture is already a factor: a spool left open for four months does not print like it did on day one unless you store it properly.

In favour there is a detail hardly anybody mentions: the end of the reel is lost once. Five one-kilo spools leave five 15 gram tails; one five-kilo spool leaves a single one. That is 60 grams recovered, a whole planter.

The rule writes itself: big formats pay off in the colour you always get through — black, grey, white — and do not pay off in the colours you use for a one-off job.

And if it runs out mid-print

It happens, and it does not always mean losing the part. Plenty of printers have a filament runout sensor: when they notice there is no strand they pause by themselves, let you load the new spool and carry on where they left off.

If yours does not have one, most slicers let you insert a pause at a specific height. Back to the example above: if you have 120 grams left and the part asks for 130, you put the pause in before you get there and change spool calmly.

What does not work is trusting that it will make it. If the sum comes out tight — and with the diameter tolerance it always comes out tight — better to start with a fresh spool and keep the open one for small parts.

What each part off that spool costs

Metres are only half the story; the other half is money. The 3D printing cost calculator splits the spool price across the grams in each part and adds power, machine and failures, and how much does 3D printing cost has twenty parts already worked out.

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