Why the ratio is a field and not a constant
Pick up two rolls of reinforcement that both say six ounces on the label and they will not take the same amount of resin. A plain weave cloth has tight bundles and very little air to fill; chopped strand mat is a pile of loose fibres held together with a binder that dissolves, and the space between them all has to become resin. Shops that weigh their panels report mat landing far higher than woven goods — often two or three times as much resin as glass by weight, against something close to one to one on a tight twill. Those are other people numbers rather than a specification, and the spread is the point: on the forty square foot job in the form, three plies of six ounce cloth is the difference between about six pounds of resin at one to one and about fifteen at two and a half.
So the field asks for your number and the hint tells you how to get it. Cut one square foot of the fabric, weigh it on a scale that reads to a gram, laminate it exactly the way you will laminate the part, let it cure, weigh it again. The weight it gained divided by the weight it started at is your ratio, by weight, for that fabric and that pair of hands. Nothing else on this page is uncertain.
By weight, and what that means
A ratio of 1 to 1 means the resin weighs the same as the glass. The finished laminate is then half resin and half glass by weight. It is not half resin and half air, and it is not one part resin to two parts total. Getting this backwards is the single most common way people end up ordering half the resin they need, because a laminate that is 50 percent resin by weight sounds like the resin is somehow already accounted for.
By volume the same laminate looks nothing like that. At the densities in the form the glass is a bit over twice the density of the cured resin, so a one to one laminate by weight works out about 69 percent resin by volume. That is why the volume figure on the result comes out of the density you typed rather than being read across from the weight ratio, and why the fibre volume fraction on the ply schedule page is a separate calculation rather than the same number rearranged.
Pot loss is not rounding
The resin that stays in the mixing pot and soaked into the roller cover is a fixed amount per batch, not a percentage. Four ounces per batch across the six batches the default job takes is twenty-four ounces — a pound and a half — and on a small job that is a bigger loss than the offcut waste. It is also the reason mixing small is expensive: cut the batch from thirty-two ounces to sixteen and the batch count doubles and the pot loss doubles with it. The page counts batches explicitly for that reason, and refuses to run if the batch size does not exceed the pot loss, because in that case nothing at all reaches the cloth.
Against that, there is a reason people mix small, and it is the same reason a big batch is dangerous: a mixed pot heats itself, and the heat has nowhere to go in the middle of a deep container. That is a genuine hazard, not a rule of thumb, and this page does not tell you how to manage it. It only counts what a chosen batch size costs you.
Where the number stops being reliable
Everything here assumes flat area. A curved surface takes more cloth than its footprint, sometimes much more, and a compound curve takes more again once the darts and relief cuts are counted. The area field wants the developed surface, not the plan view. Corners take extra resin regardless of area because the fabric bridges and the resin fills what is left. And a layup done in a cold shop soaks differently from one done warm. None of that is in the arithmetic; the ratio field is where you put it.
Questions people ask
How much resin does a square foot of 6 oz cloth take?
At the ratio in the form — 1.5 parts resin per 1 part glass by weight — one square foot of 6 oz per square yard cloth holds 0.667 oz of glass and takes 1.0 oz of resin. Three plies of it over 40 square feet, with 8 percent overlap and 10 percent offcut waste, comes to 5.94 lb of glass and 8.91 lb of resin, and once the pot and roller loss is counted you mix 10.41 lb across 6 batches. Change the ratio and every one of those numbers moves with it.
Does a 1:1 resin to glass ratio mean the laminate is half resin?
By weight, yes. A ratio of 1 means the resin weighs the same as the glass, so the cured laminate is 50 percent resin and 50 percent glass by weight. By volume it is not close to half, because cured resin is roughly half the density of E-glass — the same laminate is about two-thirds resin by volume. Every ratio field on this page is by weight and says so in the label, because the two conventions give answers that differ by a factor you will notice at the till.
Why does chopped strand mat take so much more resin than cloth?
Because of what is between the fibres. Mat is loose random fibres with a binder that dissolves in the resin, so the fibres end up floating in a lot of resin. Woven cloth has the fibres already gathered into tight bundles, and the resin only has to fill the gaps between bundles. Weigh a test panel of each and you will get two very different ratios, which is why this page asks you for the number rather than assuming one.
How do I work out my own resin to glass ratio?
Cut a piece of the fabric with a known area, weigh it dry to the gram, laminate it the way you will laminate the part, cure it and weigh it again. Divide the weight gained by the dry weight. That figure is your ratio by weight for that fabric, that resin and your technique. It is worth doing once per fabric type on the shelf, and worth redoing if you switch from brush to squeegee or from a warm shop to a cold one.
Should the overlap and the waste be one number?
No, and combining them overstates the resin. Overlapped cloth stays in the part and soaks resin. Offcut waste goes in the bin and never sees any. If you enter 18 percent as a single allowance the page treats all of it as laminate and adds resin for the offcuts too. On a job with a lot of curved trimming that error runs into pounds.
Does this page tell me how many plies my part needs?
No, and it will not. How thick a laminate has to be is a structural question that depends on the loads, the geometry, the fabric, the resin and the service the part sees, and it belongs with whoever is engineering the part. This page takes the schedule you were given or chose and tells you what it costs in cloth and resin. It has no opinion on whether the schedule is right.