One-kilo spool vs refill: comparing usable material cost

What you actually pay for: net mass plus the spool

Start by defining the items compared: the same material and color if you want an equivalent-product price comparison, followed by net mass and contents of each item. The two Prusament examples have different named formats: 1kg in the older guide and 900g in the newer one. That difference does not establish that a refill costs less than a full spool. Record price and compatibility for the actual items under consideration.

Prusa's documentation names a legacy ‘1kg’ refill for the cardboard-center design and a ‘900g’ refill for the newer plastic-center spool. Those formats identify versions in the guides. For price calculations, obtain net quantity from the listing or label of the actual item purchased instead of treating the guide name as a verified net-mass measurement. Also check what the item includes rather than assuming that every listing supplies the same support or packaging.

These two guides supply no numerical empty-spool tare. To estimate remaining filament, subtract the tare of a genuinely matching empty spool from the measured gross mass. That weighing method establishes neither moisture content nor printability of the material. A filament spool scale can support the measurement when you have a reliable tare. Do not subtract the mass of a different spool merely because it looks similar.

Two documented Prusament assemblies

According to the Prusa Knowledge Base, the original Prusament spool consists of two black plastic sides pushed onto a cardboard centerpiece. The sides can be removed and reused or recycled. The 1kg refill was designed for this spool, and Prusa describes it as a legacy product without an NFC tag.

The newer guide applies only to the plastic-center spool and requires a 900g refill. It explicitly excludes the older 1kg refill from that new design. A different assembly procedure does not, by itself, prove reverse incompatibility or classify other systems on the market. For an undocumented combination, obtain compatibility information for the exact reference instead of assuming that matching advertised mass establishes fit.

Prusa further notes that gray Prusament spools exist for advanced materials. On those spools the sides are glued to the cardboard center because some filaments need drying at high temperatures. They do not come apart like the black spools. If your pile of empties is mostly gray spools, refills are not the intended path for those references. The page on reusable filament spools and refills covers reuse in general. This page stays focused on cost.

Check compatibility before you order a refill

Examine the spool design and refill reference together. The legacy guide describes removable black sides on a cardboard center and separately identifies glued gray sides used for some advanced materials. The newer guide requires a plastic center with a tool notch and the 900g refill. Color or a cardboard center alone does not establish universal compatibility. Compare the actual assembly with the corresponding guide and refill listing before ordering.

The 900g guide says newer refill coils have used rubber bands since March 2026, while older stock may retain a paper strap. Keep that transport and procedure distinction for the batch received. It does not identify every spool generation or prove compatibility with another spool. Follow the steps matching the restraint actually supplied, rather than treating the band or strap as the only identification criterion.

If the required spool is missing, record how it can be obtained and its actual cost. These guides do not establish that a new full spool of filament must always be bought first. You may already own a compatible support, or another offer may need verification. If your scenario includes purchasing an initial full spool, state that assumption and include the filament supplied with it in the overall comparison.

How to calculate cost per usable gram correctly

For delivered material cost, use C ÷ M, where C is the selected price and M is net mass in grams. The result is currency per gram; multiply by 1,000 for cost per kilogram. If confirmed net quantity is 900g, use 900g in that calculation rather than assuming 1,000g. This editorial arithmetic compares delivered material. It neither guarantees that all supplied mass becomes useful parts nor predicts a quantity of printing losses.

Keep equipment costs separate. With an empty spool already owned, you can show the marginal refill cost and any missing tool. If a full spool is bought and then refilled, overall cost is (full-spool price + refill price + included additional costs) ÷ (full-spool net mass + refill net mass). Do not charge the whole full-spool price against only the refill's mass. The print cost tool helps estimate the printed opener using your actual inputs.

Moving from delivered cost to usable cost requires an observed usable mass and a clear definition, such as material available after known losses. Without that measurement, keep net delivered cost and the question of losses separate. Include shipping, taxes and additional costs consistently in both options. No price is supplied here. The startup budget tool helps organize the amounts actually recorded without inventing a current offer or assuming the refill must be cheaper.

Refill time and the tools you need

Handling also has a cost if you choose to value it, but the guides provide no guaranteed duration here. Time your own operation instead of presenting a few minutes as a measured fact. The legacy guide removes both sides from the old cardboard center, which is no longer needed. It then attaches the sides to the new refill bundle without removing the orange tape. Cut the strips only after firm assembly; Prusa recommends scissors rather than a knife.

For the newer plastic-center spool, the 900g refill guide is rated moderate difficulty and requires a printed tool. The Prusament Spool Opener goes into the notch in the spool center, locks when you lift its inner part until it clicks, and then turns counterclockwise to release the side. The inner filament end must go into the dedicated opening before you slide the refill onto the center.

The 900g guide says to hold the filament end while removing the transport straps to prevent tangling, then secure it to the side when it is not loaded. The legacy guide emphasizes leaving the tape until assembly is firm. Retain those instructions within their respective procedures. They do not quantify failure rates or savings erased by a tangle. For a cost comparison, record only the losses actually observed rather than treating a possible event as an inevitable expense.

When to compare a full spool as well

A refill can be considered when the compatible support is available and its assembly procedure is acceptable. Financial benefit still needs calculation using actual offers, masses and additional costs. Reusing spool sides is documented for the older system; the two guides supply no life-cycle analysis quantifying an overall environmental advantage. Keep physical reuse, budget and any losses as separate criteria rather than assuming that every refill necessarily saves money or has a lower total footprint.

When changing brands, check the support required by each refill; a brand change alone does not prove that every existing spool is incompatible. Prusa's described glued gray sides do not follow the disassembly of the older black design. If several spools must remain loaded simultaneously, count the supports required by your scenario. Do not assume that one support can hold several separate refill bundles at the same time or invent how many new full spools must be purchased.

Finally, the legacy 1kg guide describes a small paper material label on the securing strip that can be attached to the reused sides. Do not extend that packaging statement to every 900g refill without checking its own instructions. You can also record product reference, material, color and lot in an inventory. To select the polymer rather than its packaging, see filament types. This keeps a comparison of weight or packaging from overlooking the actual material required.

Sources and limits

• Prusa — legacy Prusament 1kg refill, cardboard-center spool
• Prusa — Prusament 900g refill, plastic-center spool and transport-band versions

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.