Composite Calculations

How to Calculate Layup Labor, Resin Usage, and Laminate Thickness in Composites

Work through the core composite manufacturing formulas with real units and numbers: layup labor, resin usage, infusion time, laminate thickness, and fiber volume fraction.

Start with layup labor, the number that drives most composite quotes. The method is workload divided by pace, then padded for handling. Take a mold with 55 sq ft of surface and a 12-ply schedule: that is 660 ply-sq ft of workload. A hand-layup laminator placing woven fabric runs 40 to 70 ply-sq ft per hour depending on ply size and complexity; call it 55. Base labor is 660 divided by 55, or 12.0 hours. The Composite Layup Labor calculator then applies a handling and inspection allowance of 15 to 25 percent for orientation checks, debulk staging, and traveler signoff. At 18 percent, 12.0 times 1.18 gives 14.2 hours.

Resin usage follows a rate-times-time model. Pull the mixed resin draw straight off a scale or meter, not the datasheet. For wet layup on fiberglass, 30 to 45 lb of catalyzed resin per hour is typical for a two-person team. Run 6.5 hours at 38 lb per hour and you consume 247 lb. The Fiberglass Resin Usage calculator multiplies that by catalyzed resin cost, say 5.20 dollars per lb including hardener and pigment, for 1,284 dollars of resin on the run. Sanity-check against fiber weight: a good hand laminate targets a 50/50 to 60/40 fiber-to-resin ratio, so resin pounds should roughly track fabric pounds.

Infusion fill time is volume divided by flow rate, plus a setup allowance. Estimate resin volume from laminate uptake plus feed lines and runner losses; a mid-size part might need 145 gallons. Stable flow under full vacuum, given the resin viscosity and flow media, might hold 32 gallons per hour. Base fill is 145 divided by 32, or 4.53 hours. The Resin Infusion Time calculator adds a 20 to 30 percent monitoring allowance for line setup, degassing, and exotherm watching. At 25 percent, 4.53 times 1.25 is 5.66 hours. Compare that against resin pot life; if gel hits at 5 hours, you have a problem before you mix.

Laminate thickness capacity tells you whether a tool can meet schedule. The formula multiplies thickness added per cycle by planned cycles, then derates for uptime and yield. A build adding 0.125 in per cycle over 48 planned cycles gives a gross 6.0 in of laminate. The Laminate Thickness calculator then multiplies by uptime, say 92 percent, and first-pass yield, say 96 percent: 6.0 times 0.92 times 0.96 equals 5.30 in of good, usable laminate. That 0.70 in gap between gross and good is your combined downtime and scrap loss, and it is where schedule slips hide.

Fiber volume fraction (Vf) checks consolidation. It is fiber volume divided by total cured laminate volume. Convert fiber mass to volume using density: carbon at 1.80 g per cc, E-glass at 2.55 g per cc. Suppose fiber volume is 0.56 cu in in a 1.00 cu in cured coupon; Vf is 0.56, or 56 percent when the Fiber Volume Fraction calculator applies the 100x reporting conversion. Autoclave prepreg parts target 55 to 60 percent Vf; wet layup runs lower at 35 to 45 percent. Below spec means resin-rich and heavy; above means resin-starved and prone to dry spots.

Resin mix ratio is a simple division but a costly one to get wrong. It is base resin divided by hardener or catalyst on a matched weight or volume basis. An epoxy quoted at 100 to 28 by weight means 100 parts resin to 28 parts hardener; the Resin Mix Ratio calculator returns 3.57 as the ratio, or you report it per 100 parts resin. Off-ratio batches never fully cure: a 10 percent hardener shortfall can drop glass transition temperature by 15 to 25 degrees C and leave a tacky surface. Always weigh the batch rather than eyeball pump strokes.

Tie the pieces together on a single part before you trust any one number. Convert areal weight to fiber mass first: 12 plies of 300 gsm carbon over 55 sq ft is 12 times 0.300 kg per sq m times 5.11 sq m, about 18.4 kg, or 40.6 lb of fiber. That feeds the material weight, the resin ratio check, and the Vf calculation. Chaining the calculators, layup labor at 14.2 hours, resin at the target ratio, cure capacity, and thickness, gives you a build sheet where each input is traceable to a scale, a meter, or the ply book rather than a guess.

Published 2026-07-01.