Where cured ply thickness comes from
There is no thickness printed on a roll of fabric, and there cannot be, because a dry ply has no fixed thickness — it is mostly air until somebody fills it. What the roll does tell you is areal weight: ounces per square yard, or grams per square metre. That weight divided by the density of the fibre gives a volume of solid glass per unit area, and the resin that goes with it, at whatever ratio you achieve, gives a second volume. Add the two and divide by the area and you have a thickness.
Worked through for the middle layer in the form: 24 oz per square yard of woven roving is 814 grams per square metre, which at 2.55 g/cm3 is 0.0319 cm of solid glass. At a one to one ratio by weight the resin is another 814 grams, which at 1.15 g/cm3 is 0.0708 cm. The ply comes out 0.1027 cm, or 0.040 inches. Two of them is eighty thousandths. Push the ratio to 1.4 and the same two plies measure 0.103 inches instead of 0.081 — a quarter thicker, from the same cloth, with nothing changed but how wet it went down.
Fibre volume fraction, and why it is not the weight fraction
The weight fraction and the volume fraction of the same laminate are different numbers and it is worth keeping them apart. A one to one laminate by weight is 50 percent resin by weight. But at the densities in the form the glass is a bit over twice the density of the cured resin, so the same laminate is roughly 31 percent glass and 69 percent resin by volume. If you are used to seeing fibre volume fractions quoted for infused or prepreg parts, those figures are volume fractions, and comparing them to a weight fraction off a hand layup is comparing two different quantities that happen to both be percentages.
The page prints both, computed from the densities you entered, precisely so the comparison is available without anyone having to do the conversion in their head. Change the fibre density from glass to carbon and watch the volume fraction move while the weight ratio stays where you put it.
Voids, and what the thickness assumes
The stack thickness here is glass volume plus resin volume and nothing else. A real hand layup traps air — between bundles, under a bridged corner, in the weave of a heavy roving — and that air makes the part thicker than the arithmetic says without adding any weight. So a measured thickness larger than the calculated one is normal, and the size of the gap is a rough measure of how much air is in there. Vacuum bagging pulls the number down towards the calculation; a badly wetted mat layup can sit well above it.
The one thing the gap is not is a quality score. Some voids matter enormously and some are cosmetic, and which is which depends on the part and its service. This page measures a gap. It does not grade it.
What the layer order does and does not change
Type the layers in any order and the totals do not move: thickness, weight, fibre volume and cost are all sums, and sums do not care about order. What order changes is stiffness, and it changes it a great deal, because material far from the middle of the stack does far more work in bending than material near it. That is why a surfacing mat goes on the mould face and the heavy roving goes where it goes, and why moving a layer can change a panel without changing a single number on this page.
Which is exactly why the page stops at thickness and weight. Turning a stack into a stiffness, and a stiffness into a verdict about whether the part will do its job, is engineering, and the person doing it needs the loads, the geometry and the service conditions, none of which are on this form.
Questions people ask
How thick is one ply of 6 oz fiberglass cloth?
It depends on the resin ratio, which is why it is a field. At the numbers in the form — 6 oz per square yard, 2.55 g/cm3 glass, 1.15 g/cm3 resin, 1.1 parts resin per 1 part glass by weight — one ply cures to 0.0108 inches. Drop the ratio to 0.8 and the same ply measures 0.0087 inches; push it to 1.8 and it measures 0.0157. There is no single answer and any source that gives one has quietly assumed a ratio.
What does the stack in the default form come out at?
One ply of 1.5 oz mat at 2.5:1, two plies of 24 oz woven roving at 1:1 and two plies of 6 oz cloth at 1.1:1 build 0.108 inches of cured laminate. The glass in it is 6.83 oz per square foot and the resin another 7.22, so the laminate weighs 14.05 oz per square foot, is 51.4 percent resin by weight, and works out to 29.9 percent fibre by volume. Over 40 square feet that is 17.08 lb of glass and 18.04 lb of resin, for a 35.13 lb part.
Why is my part thicker than this calculator says?
Almost always air. The calculation is glass volume plus resin volume with nothing between them, and a hand layup traps air in the weave, under bridged corners and around heavy roving. That air adds thickness without adding weight, so a measured thickness above the calculated one is the normal case rather than a fault in the arithmetic. Weigh the part and run the numbers backwards on the resin content page to see how big the gap actually is.
Is fibre volume fraction the same as glass content?
Not unless somebody says which one they mean. Glass content quoted by weight and fibre volume fraction are two different numbers for the same laminate, because glass is roughly twice the density of cured resin. A laminate that is 50 percent glass by weight is about 31 percent glass by volume. Infusion and prepreg figures are usually volume fractions and hand layup figures are usually weight fractions, so the two get compared far more often than they should be.
Does the order I list the layers in matter?
Not to anything on this page. Thickness, weight, fibre volume and cost are sums and sums do not care about order. It matters enormously to the part, because how stiff a panel is depends on how far the heavy material sits from the middle of the stack, but turning that into a number needs the loads and the geometry and is an engineering job rather than a calculator one.
Can I use this for carbon fibre?
Yes, with the fibre density changed. Carbon is lighter than E-glass, so the same areal weight fills more thickness and the fibre volume fraction lands differently. Put the density from your fabric supplier in the reinforcement density field and the rest of the arithmetic is unchanged. Aramid works the same way. What does not carry across is the resin ratio, which is a property of the weave and your technique and needs its own test panel.