The constraint nobody prices
Estimating a coating job usually stops at gallons. The number that actually decides whether the floor comes out flat is time, and it is set by three things that have nothing to do with each other: how much material is in a kit, how far that material goes, and how fast people can move it around.
Run the defaults. A 3 gallon kit at 250 square feet a gallon reaches 750 square feet. Two people spreading 30 square feet a minute each cover that in 12.5 minutes. A 30 minute working time cut to 70 percent for a warm shop is 21 minutes, and 3 minutes of that goes on mixing and induction before anything reaches the floor, leaving an 18 minute window. Twelve and a half minutes of spreading inside an 18 minute window is 5.5 minutes of margin, which is comfortable but not generous.
Take one person off and the same kit takes 25 minutes to spread inside an 18 minute window. Nothing about the product changed. The crew size did, and that is the lever that is actually available on the day.
The clock starts at mixing
Pot life is quoted from the moment the parts are combined, so the minutes spent mixing, inducting and getting the pail to the far corner are minutes off the window rather than free time before it. That is why mixing time is a deduction on this page rather than an addition. On a small batch it barely registers. On a job where each kit gets three minutes of mixing and a walk across a warehouse, it is a meaningful bite out of a twenty minute window.
The other thing the sheet figure does not capture is that a pail of mixed material is a worse place to be than a thin film on a cold slab. The reaction generates heat, the mass holds it, and the hotter it gets the faster it goes. Material spread out immediately behaves better than material that sat in the bucket while somebody found the roller frame.
Where the wet edge fails
Every batch change is a seam. The new material has to be worked into the previous batch while that batch is still open, and the previous batch has been going off since it was mixed rather than since it was poured. On a floor that needs eight batches, there are seven of these and they are all in different places.
The practical answer is layout rather than speed: plan the runs so a batch ends at a column line, a joint or a doorway, where a slight difference in the film is invisible, instead of stopping halfway across an open bay where it will be the first thing anyone sees. That is a decision made with a chalk line before the first kit is opened.
Getting the inputs
The spread rate box wants the practical figure for the film you are building, which comes out of the spread rate calculator if you have the volume solids and a target film, or off the label if you have a published coverage figure and the garage floor coating calculator to adjust it for the profile. The area for the coat should be the real coated area including cove base and pads, which is the coating area take-off. The working time itself only ever comes from the data sheet for the product in front of you.
Questions people ask
How much epoxy can I mix at once?
As much as the crew can get onto the floor inside the working time, minus what mixing and moving the pail consume. That is a different number on every job because it depends on the spread rate, on how many people are actually spreading and on how cut-up the room is. What is not a good answer is a whole kit because a whole kit is what the box contains. This page turns it round: it tells you how much floor is reachable in the window you have, converts that to gallons, and shows it as a share of one kit so you can see whether a full kit fits or whether it needs more people.
Does pot life really change that much with temperature?
Enough that planning off the sheet figure without adjusting it is a common way to lose a batch. The sheet quotes a working time at a stated temperature and the reaction speeds up as it gets warmer, so a hot shop in July is not the conditions the number was measured in. The mass matters as well: a full pail retains the heat the reaction generates and accelerates itself, while the same material spread thin on a cold slab behaves far better. This page keeps that as an allowance percentage you set rather than a rule, because the honest figure is your own record on the same product in your own building.
Can I mix half a kit?
Sometimes, and it is a question for the data sheet rather than for a calculator. Two-part products come proportioned so that emptying both containers gives the correct ratio, and splitting a kit means you are measuring the ratio yourself. Some products tolerate that with an accurate scale or graduated container and some state plainly that they should not be split, because being off ratio does not produce a slightly worse coating, it produces one that never cures properly. If splitting is not permitted, the levers left are more people spreading, a thinner film, or breaking the floor into areas that can be finished at a natural stopping line.
What spreading speed should I use?
The default of 30 square feet a minute per person is a squeegee-and-back-roll figure on open floor and it is optimistic for most real rooms. A garage with cabinets down one side, a floor drain, a step at the door and a cove base to cut in first is not open floor. The reliable way to get the number is to mark out a known area during a previous job and time it, and if you have never done that, run the page twice with a fast figure and a slow one and plan for the slow one. Being wrong in the optimistic direction is the expensive way round.
What happens if the batch goes off before it is spread?
It stops flowing and starts dragging, the film thickness goes wherever the material happens to be, and the seam with the next batch stops being a seam and becomes a visible ridge. Trying to rescue it by thinning is generally worse than living with it, because thinning changes the ratio and the cure. The material that is left in the pail is waste, and a full pail of mixed two-part material generating its own heat as it goes off is a hazard rather than merely a loss, which is a good reason to mix less rather than more when the margin looks thin.