Solids by Volume and What a Coating Really Covers
Price per gallon tells you almost nothing. Solids by volume is the number that decides how far a product goes, what thickness you can hit, and whether the quote you just sent makes money.
Solids by volume is the share of what is in the tin that stays on the floor. It decides how far a gallon goes, so it decides cost per square foot. At a 10 mil dry film target a 100% solids product covers 160.4 ft²/gal and a 40% solids product covers 64.2, which means the cheaper tin needs 2.5 times the material to build the same floor. Compare products on cost per square foot at the same thickness, never on price per gallon. Work yours out in the film thickness calculator.
What the number means
A coating is solid matter carried in something that leaves. Solids by volume is the proportion that stays. Lay a 60% solids product 10 mil thick and you have 6 mil of floor once it has cured; the other 4 mil went into the air. That is the whole idea, and everything practical follows from it.
At 100% solids nothing leaves, so wet and dry thickness are the same number and the distinction never comes up. That is exactly why it catches people out when they move to a product that is not 100% solids: the habit of treating the two as interchangeable was never wrong before.
Volume solids, not weight solids
Data sheets often carry both. Only one of them predicts film thickness, and picking the wrong one is expensive in a specific direction: it makes the product look like it goes further than it does, so you order short.
The reason is that the solid part of a coating is usually denser than the liquid part, so it accounts for more of the weight than it does of the volume. A product can be 80% solids by weight and 65% by volume. Film thickness is a volume question, so volume solids is the answer. If a sheet gives a single unlabelled percentage, treat that as a question for the manufacturer rather than a number to use.
Wet film is the one you can control
You cannot measure dry film thickness until the coat has cured, at which point knowing it is a matter of record rather than a matter of doing something. Wet film you can measure while the product is still workable, with a gauge that costs very little, and correct on the spot.
The conversion is the solids fraction. To land 10 mil dry from a 70% solids product you apply 14.3 mil wet. Set the gauge to the wet number, check it on the first bay rather than the last, and check it again when the crew changes or the roller does.
Work out your own coverage and wet film
Solids and a target thickness in; theoretical coverage, coverage after losses, the wet film to apply, and a volume to order out. It runs backward too, so you can audit a coat that is already down.
Returns 140.4 ft²/gal theoretical, 126.3 practical, and 11.4 mil of wet film to apply
Open the live calculator →Why the data sheet coverage never happens
The figure on the sheet is theoretical coverage, and it assumes every drop lands on the floor at exactly the target thickness. Real jobs lose product in ways that are entirely predictable and still routinely left out of quotes.
Where it goes
- What stays in the bucket and on the roller or squeegee
- What an open, porous or freshly ground slab absorbs
- Film pooling thicker in low spots than the average implies
- Edges, cut ins, columns and doorways
What to allow
- 10% is a common starting point, not a rule
- More on a rough or porous slab, especially a first coat
- More on a floor with a lot of boundary relative to its area
- Less on a tight, sealed floor being recoated
The honest version of this is to record what you actually used against what you estimated, job after job, until the allowance is your own figure rather than a borrowed one. Until then, 10% and a note to check.
Comparing two products without fooling yourself
Price per gallon is close to meaningless on its own, because it says nothing about how many gallons the job needs. The comparison that means something is cost per square foot, at the same finished thickness, including the number of coats each product needs to get there.
Do it in three steps. Read each product’s coverage at the dry film thickness your system actually requires. Divide the price per gallon by that coverage to get cost per square foot per coat. Then multiply by the coats each one needs to reach the finished film.
That third step is the one people skip, and it is often the one that decides it. A lower solids product frequently needs an extra coat to build the same thickness, and an extra coat is not only more material: it is another day on site, another mobilisation, and another recoat window to hit. The coverage per mil chart lets you read both products off the same grid without doing the arithmetic twice.
Auditing a coat that is already down
You do not have to wait for a thickness gauge to find out whether a floor got what was specified. Divide the area you covered by the product you used and you have your real spread rate; put that into the calculator in measured spread mode and it returns the dry film that rate produces.
Do it on the first bay. A crew running 200 ft²/gal on a 100% solids product is laying 8.0 mil, and if the specification said 10 the floor is a fifth thin before anyone has looked at it. Caught on the first bay that is a conversation about pace; caught at the end it is a conversation about a recoat.
Where this sits in a quote
Coverage is the hinge between a data sheet and a price. Get it wrong and every downstream number is wrong with it: material cost, the number of kits, the delivery, and the margin you thought you had. Get it right and the material side of a quote becomes arithmetic rather than a guess.
Once you have a quantity, the epoxy coverage calculator turns it into whole kits and the Part A and Part B split, and the cost per square foot guide shows where material sits against labour in a finished job.
Frequently Asked Questions
What does solids by volume actually mean?
It is the proportion of the liquid in the tin that stays on the floor. A 60% solids coating laid down as 10 units of wet film leaves 6 units of cured film; the other 4 evaporate as solvent or water.
That is the whole concept, and every practical consequence follows from it: how far a gallon goes, how thick you have to lay it, how many coats a system needs, and what it really costs per square foot.
Why is solids by weight the wrong number to use?
Because film thickness is a question about volume, and weight solids answers a different question. The solid part of a coating is normally denser than the liquid part, so the weight figure comes out higher, sometimes much higher.
Use it in a coverage calculation and the product looks like it goes further than it does, so you order short and the job stops. Data sheets usually list both and label them. If a sheet gives one unlabelled percentage, do not guess which it is.
Is 100% solids always the better buy?
Not always, but it is almost always the better value per square foot, which is the comparison that matters. It also builds film in one pass, so a thick system needs fewer coats and fewer days.
The cases against it are real though. It is less forgiving to apply, because there is no solvent to help it flow and the pot life is often short. Some jobs genuinely want a thin penetrating coat that a high solids product cannot give. And a few specifications call for a low viscosity primer precisely because it soaks in. Choose on the job, but do the cost per square foot arithmetic first rather than reading the price per gallon.
How do I compare two products properly?
Divide, do not glance. Work out coverage at the same dry film thickness for both, then divide the price per gallon by that coverage to get cost per square foot per coat. Then multiply by the number of coats each system needs to reach the same finished thickness.
That last step catches the cases the first one misses. A cheaper low solids product frequently needs an extra coat to build the same film, and an extra coat is not just more material: it is another day, another mobilisation, and another recoat window to get right.
What loss allowance should I use?
Ten percent is the usual starting point and it is a starting point, not a rule. Unlike coverage arithmetic, this one is a judgement.
Go higher on a rough, porous, or freshly ground slab, because the first coat will soak in. Go higher on a floor with a lot of edges, cut ins, columns and doorways relative to its area, because the waste is at the boundaries. You can go lower on a tight, sealed, previously coated floor being recoated. The honest approach is to record what you actually used against what you estimated, job after job, until you have your own number instead of a borrowed one.
My floor measured thinner than the spec. What went wrong?
Usually one of three things, and the calculator will tell you which. Work out the spread rate you actually achieved, area covered divided by product used, and put it into measured spread mode.
If the achieved thickness is well under target, you spread it too far, which is the common one and is fixable by slowing down and checking with a wet film gauge. If the achieved thickness looks right but the floor measures thin, you may have used a weight solids figure somewhere, or the slab drank more of the first coat than the loss allowance assumed. If the numbers all agree and the floor is still thin, measure in more places, because an average hides a thin bay.