The slicer’s weight estimate is the best starting point for a specific model. Calendar time depends on what you print; filament length depends on diameter and material density; usable mass depends on purge, handling, and failures.

Three meanings of “last”

  1. Number of completed prints: useful for repeat products, classroom sets, or batches.
  2. Printer-hours: useful for capacity planning when the same part repeats.
  3. Calendar time: useful for inventory, based on your actual weekly or monthly consumption.

Keep the question explicit. A spool can last 500 printer-hours on tiny slow parts or disappear in a few large high-flow prints.

Estimate complete prints from grams

Use the sliced job total, including supports and adhesion features visible in the preview.

adjusted grams per job = slicer grams × (1 + overhead percentage ÷ 100)
complete jobs = floor(1,000 ÷ adjusted grams per job)

Example

A model uses 20 g in the slicer. You add a 10% allowance for purge and handling:

20 × 1.10 = 22 g per completed job
floor(1,000 ÷ 22) = 45 complete jobs

This is arithmetic, not a measured production result. It leaves approximately 10 g after 45 planned jobs, and it assumes the spool contains the stated net filament mass.

Estimate printer-hours

Multiply the number of complete jobs by the slicer’s duration estimate:

completed print-hours = complete jobs × hours per job

If the 45 example jobs each take 2.5 hours, the plan contains 112.5 completed print-hours. Actual elapsed machine time can include warm-up, calibration, cooldown, pauses, maintenance, and retries that do not consume filament at the same rate.

For cost—not only duration—use the 3D Print Cost Calculator.

What the slicer may not include

Check whether its reported total includes:

  • purge or flush material during color changes;
  • a wipe or prime tower;
  • skirt, brim, or raft;
  • supports and interface layers;
  • startup purge lines or blobs;
  • filament used while loading and unloading;
  • calibration patterns printed automatically;
  • failed prints and discarded test pieces;
  • an unusable final tail near the spool core.

Multi-color waste can exceed the model itself in some workflows. Use the slicer’s complete filament breakdown and an actual purge strategy rather than a generic percentage.

Convert 1 kg to approximate length

Length depends on filament diameter and density. The calculation treats filament as a uniform cylinder:

cross-sectional area (cm²) = π × (diameter in mm ÷ 20)²
volume (cm³) = mass in grams ÷ density in g/cm³
length (m) = volume ÷ area ÷ 100

At 1.75 mm diameter and 1.24 g/cm³ density, 1,000 g is approximately 335 m. The 1.24 value is specific to the referenced Prusament PLA technical data—not a constant for all PLA colors and blends.

Use the interactive Filament Calculator to change diameter, material preset, or density.

Improve the estimate

Use the actual product density

Look for density in the manufacturer’s technical data sheet. A generic preset is appropriate only for a rough estimate. Filled, foaming, recycled, or modified products can differ meaningfully.

Use a measured diameter carefully

The nominal diameter is normally enough for planning. If high precision matters, use an appropriate instrument and multiple readings around and along the filament. Do not overfit a calculator to one squeezed or angled measurement.

Track real overhead

Record the starting spool mass, empty spool tare, job estimates, purge, and failed output across several jobs. This produces an evidence-based allowance for your workflow instead of an invented “failure rate.”

Keep spare capacity

Do not schedule the final gram of a spool for a critical unattended job. A nominal 1 kg describes net product, while spool shape, sensor position, path friction, and the final bend can leave some material impractical to use.

Worked example for a partly used spool

Suppose the spool and filament together weigh 420 g, and the known empty-spool tare is 180 g. The estimated filament remaining is 240 g. If you reserve 20 g for an unusable tail and uncertainty, 220 g remains available for planning. A sliced job reporting 30 g of model, supports, brim, and purge allows seven complete planned jobs: floor(220 ÷ 30). These are invented inputs demonstrating arithmetic, not measured Bambu production data.

Do not subtract a tare borrowed from a different spool design. If the tare is unknown, mark the remaining mass as uncertain instead of presenting gross spool weight as usable filament. Check whether the slicer total already includes flushing and towers before adding another allowance. If a shared Bambu project changes color assignments or purge settings, re-slice and compare the full breakdown again.

Calendar consumption

Divide usable mass by your observed consumption per week or month:

weeks remaining = usable grams ÷ average grams used per week

Use a rolling average that matches the work you expect. A month of miniature prototypes is not a reliable forecast for a month of large fixtures.

Label opened spools and keep them sealed as described in How to Store 3D Printer Filament. Inventory age alone does not show moisture condition.

Source and scope

The numerical density example uses the Prusament PLA technical data sheet. Use the density and tolerance published for the exact product when accuracy matters.