Why a part twice the size costs four times as much

You type 200% into the scale box in your slicer and the weight goes from 60 to 480 grams. Eight times the plastic. The natural reaction is to assume the part will cost eight times as much.

It costs four. From 5.11 € to 19.88 €.

And the other way round something worse happens: shrink it by half and you use an eighth of the plastic, and you save 36%. This article explains both.

Volume goes up with the cube, and that part is exact

When you scale a part to 200% you are not making it twice as big in one direction: you are making it twice as wide, as tall and as deep. Two by two by two is eight.

That part is not an estimate, it is geometry: multiply every dimension by a number and the volume multiplies by that number cubed. And the weight follows the volume.

Fifty per cent more scale is already three and a half times the plastic. At 300%, twenty-seven times: 1.6 kilos from a part that started at sixty grams.

In this table the time also goes up with the cube, because on a part with infill the time tracks the volume. There is a whole section further down on when that does not hold.

The cost does not go up with the cube

Here is what breaks the intuition. The plastic multiplies by eight, but the cost of the part does not:

The reason is the usual one on this site: your time does not scale. Taking the part off, cleaning the bed and packing it costs the same quarter of an hour whether the part weighs seven grams or half a kilo.

And look at 150%: that is where the cost multiplies almost exactly by two. If somebody asks for “twice as expensive”, that is not twice the size, it is 50% more scale.

At double size, your time stops being in charge

The normal part has a very lopsided breakdown: your time is 59% of the cost and the material only 23%. At double size it turns over:

Your quarter of an hour has gone from 59% to 15% without you changing anything you do. Now the plastic is in charge, and for the first time it makes sense to fight over the price of the spool.

You can see it yourself in the calculator with the part at 200%.

Shrinking a part barely saves anything

This is the other face, and in practice it is the one that costs the most money, because people scale down assuming it gets much cheaper.

At 50% the part weighs 7.5 grams and takes 41 minutes. The plastic drops from 1.20 € to 15 cents. And the total cost drops from 5.11 € to 3.26 €: 36% less, not 87%.

The breakdown explains why: at that size your time is 92% of the cost and the material 4.6%. You are no longer selling plastic, you are selling the time spent attending to the printer.

Check it in the calculator with the part at 50%: the material disappears from the chart.

The practical conclusion is uncomfortable but useful: on small parts, cutting the price because “it is tiny” is giving away your time. A 7.5 gram part does not cost eight times less than a 60 gram one; it costs a third less.

The limit is not the money: it is the spool and the clock

Before taking on a scaled-up job, look at these two columns, because they usually decide more than the price does:

At 250% you eat a whole spool in a single part. At 300% you need more than a spool and a half, and if the reel runs out mid-print in a colour you cannot match, you throw away 148 hours.

And those 148 hours are twelve days with the machine running twelve hours a day. Twelve days in which you print nothing else. That does not show in the price of the part, but it shows in the month's break-even point.

There is also an obvious physical limit: at 300% the part is three times the size too, and it may not fit on your bed. You see that before any sum.

One big part or eight small ones

The 44 hours of the part at 200% are not just a lead time: they are eight normal parts you are not going to print. It is worth seeing which comes out better.

The eight small ones leave twice the profit: 20.44 € against 9.94 €. Same machine, same hours.

But look at the last row before deciding. Those extra 10.50 € cost you an hour and three quarters of your own time, taking off, cleaning and packing seven more times. That works out at 6.00 € an hour: half what your hour is worth in this very calculator.

So the answer depends on what you have spare and what you are short of. If the printer is the scarce thing, the eight small ones win easily. If you are the scarce thing, the big part pays you better for every minute you give it. And if you sell both, you now know which to put in the queue when the machine is free and you are not.

When the time does not go up with the cube

The whole table assumes the time scales like the volume. On a solid part or one with high infill that holds fairly well, because nearly all the time goes into filling.

On a hollow or thin-walled part, it does not. There the head mostly traces perimeters, and perimeters grow with the surface, which goes up with the square and not the cube. If your part is that kind, the time at 200% multiplies by four rather than eight:

The difference between the two is 3.16 €, less than the material changes by. Which means: even if you get this assumption backwards, the conclusion of the article does not move.

And in any case there is no need to assume: slice the scaled part and look at the time your slicer gives you. That figure is yours and it is exact.

Stretching is not the same as scaling

There is a very common case that does not follow the cube rule: when you do not enlarge the whole part but stretch it along a single axis. A taller bracket, a longer spacer, a deeper box.

There you multiply only one dimension, so the volume rises in the same proportion, not cubed:

Compare the middle row with the earlier table: twice as tall costs 7.22 €; twice everything costs 19.88 €. Nearly three times less, and both are said just as quickly on the phone.

That is why it is worth asking what exactly the customer means by “bigger”. If what they need is for something to fit inside, stretching one axis is nearly always enough, and that changes the quote completely.

Scaling also scales the holes

This is not a money problem, but it wrecks half the scaled parts out there, so it is worth saying before you take the job on.

When you scale, everything scales. A hole meant for an M3 screw is 3 mm; at 200% it is 6 mm, and an M3 no longer fits. The same goes for a bearing seat, a thread pitch, a shaft diameter or the clearance on a lid that used to be a snug fit.

Walls grow too. A 1.2 mm wall designed for three perimeters becomes 2.4 mm, which breaks nothing but does change the time and the weight you calculated.

Downwards it is worse: at 50%, that wall ends up at 0.6 mm. If your nozzle is 0.4 mm, the slicer has to decide what to do with half a perimeter, and what usually happens is the part comes out fragile, or hollow where it should not be.

The practical rule: scaling is safe on decorative parts and risky on parts that have to fit something. If the part involves screws, magnets or bearings, do not scale it: remake it at the new dimensions and charge that time as design time.

What to do with this when you quote

Do not multiply the price by the change of scale. That is the direct mistake. If they ask for the part at double and you charge double, you lose: it costs nearly four times as much.

Do not multiply by eight either. That is the opposite mistake and it also happens: you get a price of 61 € for something that costs 19.88 € to produce, and the customer walks.

Charge small parts what they are worth. A 7.5 gram miniature costs 3.26 €, not 64 cents. If you sell it for a euro you are paying for the privilege of working.

Set a minimum order. It is the natural consequence of the above: below a certain size the cost stops depending on the part and becomes almost entirely your time, and that floor is worth writing down.

Look at the hours before the price. A job at 300% occupies the machine for twelve days. If it pays well but leaves you without a printer for two weeks, it is still bad business, and that comes out in the sums in how to quote a job.

Do it with your own figures

The shortcut is to trust no rule and let the slicer give you both numbers.

One. Slice the part at the size they are asking for, not the original one, and write down the grams and the time it reports.

Two. Put them into the cost calculator, leaving your handling minutes exactly as always: that is precisely the field that must not change with the scale.

If you cannot slice it yet, the cube rule gives you a reasonable estimate: multiply the grams by the scale cubed. At 130% that is 2.2 times; at 170%, 4.9 times.

And if what changes is not the size but the material, the effect is far smaller: it is worked out in what PETG, ABS or TPU cost.

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