Estimating Filament Remaining: Never Run Out Mid-Print
Mathematics and tricks for managing your spool inventory.
The nightmare of every 3D printer owner is the 'spool run-out' at 95% of a 2-day print. While some modern printers have run-out sensors, many don't, and even those that do require you to be there to swap spools. Knowing exactly how much material is left on your spool—either by weight or by measuring the remaining rings—is an essential skill for any serious maker.
The Weight Method (The Most Accurate)
The easiest way to know what's left is to weigh the spool on a kitchen scale. Subtract the 'Tare Weight' (the weight of the empty plastic or cardboard spool) from the total. Most standard 1kg spools weigh between 200g and 300g empty. If your total weight is 450g and the empty spool is 250g, you have 200g of usable filament left.
Length to Weight Conversion
Slicers like Cura usually tell you the required 'Length' in meters. To convert the weight we found (200g) back into meters, you need the density of the material. For 1.75mm PLA, 1 meter weighs approx 3 grams. So 200g is about 66 meters. Our online calculator makes this conversion instant for any material type or diameter.
The 'Empty Spool' Knowledge Base
To use the weight method, you must know the empty spool weight. It's a great habit to weigh a new, full spool and subtract 1000g, or weigh an empty spool once you finish it and write the weight on the side with a marker. Common weights: Prusament (Plastic: 200g), Hatchbox (Plastic: 220g), eSUN (Plastic: 250g), Overture (Cardboard: 170g).
Visual Estimation and Spool Gauges
If you don't have a scale, some spools have a clear 'gauge' on the side showing remaining grams. These are notoriously inaccurate. For a better visual check, compare the radius of the remaining filament to the total spool radius. Because it's a volume calculation (Area = πr²), when the spool looks 'half full' by radius, it's actually much less than half full by weight!
FAQ
Can I join two pieces of filament together?
Yes, with a 'filament fuser' or by carefully melting the ends and pressing them together, but it is risky. It's usually better to use a run-out sensor or pause the print manually to swap the material.
How much 'safety margin' should I leave?
Always leave at least 5-10% more filament than the slicer estimates. Slicers often under-calculate the material used in purge lines, skirts, and retractions.
Is cardboard better than plastic for spools?
Environmentally, yes. But cardboard spools can be dusty and sometimes have more friction on spool holders. They also absorb moisture, which can throw off your weight-based estimations slightly.