How many parts fit on a standard sprue?

It depends entirely on part size, not on a fixed number. A 1:35 armour sprue might carry 20-40 small fittings; a sprue holding large hull or fuselage halves may have only 6-12 parts. Tiny detail frames (clear parts, photo-etch-replacement plastic) can exceed 40. Enter the realistic parts-per-sprue figure for your kit (count it from a typical frame) rather than assuming a standard.

What determines parts per sprue?

Part size relative to the mould cavity, gate positions, runner routing and the need for draft angles and clean ejection. Larger parts crowd a frame quickly, so a frame of wings or hull halves holds far fewer parts than a frame of small fittings. The moulder also balances flow length, so very small parts cluster near the gate.

What is runner waste?

Runner waste is the plastic in the sprue frame, sprue and gates that links the parts but gets cut off and discarded. In cold-runner moulding (used for virtually all hobby kits) this is typically 10-30% of the shot weight, and it climbs toward 50% or more when a frame carries many small parts, because there is more runner and more gates per gram of usable part. The figures here are an indicative band, not a measured weight for your specific kit.

How does scale affect the number of sprues?

Indirectly, through part count and part size. A large-scale kit (say 1:16 or 1:24) of a given subject usually has bigger parts and often a higher part count, so it spreads across more frames than a small-scale (1:72) version of the same thing. There is no fixed scale multiplier — count the parts and the realistic parts-per-frame for the actual kit and let those two figures drive the layout.

ModelCalcs

Sprue Layout Calculator — Parts Per Sprue Planning

Work out how many sprues your kit needs and how many parts land on the final frame.

Total Parts

Count from the instruction sheet's part list (it usually totals them)

Parts Per Sprue

Count the parts on one representative frame — small-fitting frames carry more, hull/wing frames fewer

How We Calculate This

This calculator packs your total part count into sprues (runner frames) using your parts-per-sprue limit, then reports how full the last frame is.

How it works

Sprues needed = ⌈ Total parts ÷ Parts per sprue ⌉ (rounded up)
Parts on last sprue = Total parts − (Sprues − 1) × Parts per sprue
Average per sprue = Total parts ÷ Sprues needed

The runner-waste figure is an indicative band drawn from cold-runner moulding norms (≈10-30% of shot weight, higher for frames of many small parts). It is a guide to how much plastic ends up as discarded frame, not a measured weight for your kit — that would require the real part and runner geometry.

Frequently Asked Questions

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Last updated: June 2026

All calculations are estimates. Always verify quantities before purchasing materials.