The calculator
Runs in your browser. Metric or US units at the top.
How the layup is worked out
Plies. Divide the cured thickness wanted by the cured thickness of one ply and round up: 2.0 mm of 0.25 mm plies is 8. Use the cured figure from the fabric or prepreg data, not the dry fabric thickness.
Fibre. Areal weight × area × plies: 300 g/m² × 0.5 m² × 8 = 1,200 g of dry fabric.
Resin. For a resin content R by weight, resin = fibre × R / (100 − R). The FAA Airframe Handbook (FAA-H-8083-31, Chapter 7): “The ratio of 60:40 fiber to resin ratio is considered optimum.” At 40 % resin, 1,200 g of fabric takes 800 g. The Handbook also notes prepregs with 32 to 35 percent resin content as typically no-bleed systems.
Worked example
2.0 mm cured, 0.25 mm per ply, 300 g/m² fabric, 0.5 m², 40 % resin, 10 % lost in the cup.
- Plies = ceil(2.0 / 0.25) = 8, cured 2.00 mm.
- Fibre = 300 × 0.5 × 8 = 1,200 g.
- Resin = 1,200 × 40 / 60 = 800 g; to mix: 800 / 0.9 = 889 g.
Eight plies and about 890 g of mixed resin. Split it with the mix ratio calculator.
Limits — when NOT to use this
- Structural repairs follow the SRM: ply count, orientation and material come from the repair data, not from a target thickness.
- Cured ply thickness depends on the process. The same fabric cures thinner under vacuum than by hand. Use the figure for your process.
- Bleeder plies remove resin. If the process bleeds, mix for what the part keeps plus what the bleeder takes.
Questions people actually ask
Is resin content by weight or volume?
By weight here, as fabric and prepreg data usually give it. Fibre volume fraction is a different figure.
What is oz/yd²?
The US unit of fabric areal weight: 1 oz/yd² = 33.9 g/m². A 9 oz glass cloth is about 305 g/m².
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 7 — “The ratio of 60:40 fiber to resin ratio is considered optimum” (resin rich and resin starved); “Prepreg systems with 32 to 35 percent resin content are typically no-bleed systems”.