Sprue Layout Calculator — Parts Per Sprue Planning

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.

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.

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.

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.