Volume tells you what to buy, weight tells you what to mix
A repair consumes a volume: the length by the width by the average depth. That is the figure that decides whether a quart is enough. The hardener ratio, though, is almost always written as a percentage by weight, which is why the calculator turns the volume into pounds using the filler density before it works out the hardener. Skip that step and the ratio is wrong by whatever the density happens to be.
The word doing the most work in the volume calculation is average. A dished repair is deepest in the middle and feathers out to nothing at the edges, so its average thickness is somewhere between a third and a half of the deepest point. Enter the deepest measurement as though it were uniform and the answer comes out two to three times too large, which is a cheap mistake in one direction and an expensive one in the other when you are the person who mixed it.
Where the primer film actually goes
Primer surfacer is unusual among coatings in that most of it is meant to be removed. It is sprayed to build a film thick enough to sand flat, and then it is sanded flat, and what remains is whatever the blocking did not take. The calculator makes that explicit because the number people care about is the film left, not the film applied.
| Step | Default numbers | Result |
|---|---|---|
| Film per coat | 55% solids, 440 sq ft per gal | 2.0 mils dry |
| Three coats | 6.0 mils built | |
| Blocking | 2.5 mils removed | 3.5 mils left |
The film per coat is not a separate guess. It comes out of the coverage and the volume solids: a gallon spread one mil thick covers 1,604 square feet, so a material at 55 percent solids covering 440 square feet a gallon must be laying down 1,604 times 0.55 divided by 440, which is two mils. If a data sheet gives you coverage and solids that do not produce a sensible film, one of the three numbers is being quoted for a different reduction than you think.
Breaking through is the failure this page is trying to prevent
Block until the film left is zero and the abrasive is now cutting the filler, or the metal, or the etch primer, in whichever spot happened to be highest. The panel will look uniform and it will not be, and the boundary shows up under colour as a ring or a shadow that no amount of extra clear will hide. The arithmetic here will not stop you doing it, but it will tell you before you start whether three coats was ever going to survive the blocking you had planned.
The other direction has its own failure. A heavy primer build holds solvent longer than the flash schedule allows for, and the solvent leaves later, through whatever is on top of it. That is where dieback and wrinkling come from, and it is why data sheets cap the coats applied in one session rather than leaving it to judgement.
What is not on this page
How much metal work should happen before filler is a decision about the repair, and it is not one that arithmetic reaches. Filler is a shaping material over sound, correctly formed metal. Damage that has moved structure, anything involving a welded panel, and anything at all near airbag modules, seatbelt pretensioners, or the sensors and cameras that driver assistance systems use, is collision repair with measurement, documented procedures and calibration attached to it. This page has no view on whether a repair is sound and cannot give you one.
Abrasives are the other omission. Discs, sheets and the grit sequence are a separate consumable calculation, and the grit progression calculator handles the steps and the time. Colour and clear volumes belong to the auto paint material calculator.
Questions people ask
Why does the hardener come out in grams when the filler is in ounces?
Because the ratio on the can is by weight and volume is what the repair consumes. Grams are simply the practical unit for weighing out a few percent of a small batch, and a cheap kitchen scale resolves them. If you would rather work by eye, the useful sanity check is that two percent of a mixed batch is a very small amount of hardener, and the visual guides people use are approximations that get less reliable as the batch gets smaller.
How do I estimate the average thickness of a dent?
Lay a straightedge across the low area and measure the gap at the deepest point, then take a third to a half of it depending on the shape. A sharp crease that drops away quickly is nearer a third. A broad, shallow dish is nearer a half. What you should not do is use the deepest reading as though the whole area were that deep, which is the most common way this calculation goes wrong.
What if the data sheet gives coverage but not volume solids?
Then work backwards. If the sheet states a dry film per coat instead, the volume solids is that film times the coverage divided by 1,604. If it states neither, you have coverage only, and the calculator will still give you a sensible volume to buy while the film numbers stay a guess. Manufacturers publish solids for their primers more often than not, sometimes on the safety data sheet rather than the technical one.
Does the primer figure include a sealer coat?
No. Sealer is a separate material with different coverage and a different purpose, and it is not sanded, so the film it leaves is the film it applies. Run the primer numbers here for the build coats and treat sealer as its own small calculation. Mixing them produces a film total that is wrong for both, and since the sealer coat is the one directly under the colour, its thickness is the one that has to be uniform.
Can I use this for a spot repair on a plastic bumper cover?
The volume arithmetic works the same way, but the materials do not. Flexible substrates take flexible fillers and adhesion promoters chosen for the specific plastic, and the ordinary polyester filler figures here do not describe them. The plastic type is normally moulded into the back of the cover, and it decides which product line applies. Put that product data into the fields rather than the defaults and the calculation is still valid.