Two numbers, twenty times apart
Take the default job on this page: 320 square feet at 90 square feet an hour is three and a half hours at the nozzle, and at 12 pounds a minute that is 2,560 pounds of abrasive passing through the nozzle. That figure is the same whichever medium you use. It is set by the metering valve and the clock, and nothing else.
What changes is what happens to those 2,560 pounds afterwards. Expendable media makes one trip: you bought 2,560 pounds and you now have 2,560 pounds of waste, which is 52 bags. Recyclable media at 5 percent breakdown makes an average of twenty trips before a grain is too small to work, so the same job consumes 128 pounds of new abrasive, which is three bags. Same steel, same nozzle, same hours, and a factor of twenty between the two invoices.
That is the single most useful thing to know about blast consumables, and it is why recovery floors and separators exist in shops that would never justify them on tidiness. It also explains why abrasive cost per square foot is a meaningless number to quote between shops without saying which of the two worlds you are in.
Breakdown is a rate, not a property
The breakdown percentage is the share of the media passing the nozzle that comes back unusable. Invert it and you get the average number of trips a grain makes: 5 percent is twenty trips, 2 percent is fifty, 10 percent is ten. The relationship is sharply non-linear at the good end, so the difference between 2 and 5 percent is worth more than the difference between 10 and 20.
It is not a fixed property of the medium. It moves with the pressure you blast at, with the hardness of what you are hitting, with how much the media has already been recycled, and with how well the separator is pulling fines out. A harder substrate breaks abrasive faster. Higher pressure breaks abrasive faster. This is why the only breakdown figure worth putting into the box is your own: weigh what you add to the pot over a week, count the nozzle hours, and divide.
Where the estimate goes wrong
| Assumption | What actually happens |
|---|---|
| Blast rate from a flat-plate figure | Structural steel with angle, gussets and back-to-back sections runs a fraction of the plate rate, because the nozzle is rarely square to the work |
| Media flow from the pot rating | The metering valve setting decides it, and a valve opened too far burns abrasive without cleaning faster |
| Nozzle hours equals hours on site | Trigger time is often under half the day once staging, blow-down and containment are counted |
| Breakdown from a supplier chart | Chart figures are measured on a test rig at a fixed pressure against a fixed substrate, which is not your job |
Where this sits
The air side of the same job is on the blast nozzle air demand calculator, which is where the compressor bill comes from and where a worn nozzle shows up. If the surface you are preparing is a floor rather than fabricated steel, the concrete grinding and shot blast production calculator works from a machine width and a walking speed instead. Once the steel is clean, the area you just blasted is the same area the coating has to cover: the fabricated steel surface area calculator builds it from a bill of materials, and the powder usage calculator turns it into pounds of powder.
Questions people ask
Why does the media flow not depend on the nozzle size?
Because the nozzle meters air, and the valve under the pot meters abrasive. They are two separate controls and you can set them independently, which is exactly how people end up burning twice the abrasive for the same cleaning rate. Open the valve too far and the stream goes dense, the grains interfere with each other, and cleaning slows down while consumption rises. The classic check is to blast a marked square at several valve settings and time each one. The setting that clears it fastest is almost never the densest one, and the pound-per-minute figure at that setting is what belongs in the box on this page.
What blast rate should I put in?
Whatever you measured, and if you have not measured, expect a wide spread. The rate depends on the nozzle size, the working pressure, the standoff, the angle, what is coming off and what profile you have to leave behind. Stripping loose scale is fast. Stripping a sound multi-coat system to bare steel is slow, and the last ten percent of a stubborn coating takes longer than the first ninety. Open sections of angle and channel are much slower per square foot than plate because the nozzle cannot be square to the surface for most of the pass. Mark a square yard on the real work, time it, and use that.
Does this work for a blast cabinet?
The arithmetic does. Enter the media flow the cabinet gun delivers and the breakdown rate you observe, and the pounds come out the same way. What will not carry over is the blast rate: cabinet work is usually small parts where the meaningful figure is parts per hour rather than square feet per hour, so work out the coatable area per part first and multiply. Cabinets also tend to run lower pressure and to recycle far more aggressively than site work, which pushes the breakdown rate down and the passes per grain up.
How do I get the pounds of abrasive I have to dispose of?
It is the new abrasive you added plus everything you removed from the steel, which is what the waste section works out when you fill in the coating figure. The abrasive does not vanish when it breaks down, it just becomes fines and dust that the separator pulls out, so on a recyclable system the waste tonnage and the purchase tonnage converge in steady state. What you may do with that waste is not a question this page answers. Old coatings can be lead-bearing and the classification of the waste depends on what is in it, which is a matter for your state and federal environmental agency and for testing, not for a calculator.
Why is the day count higher than I expected?
Because the hours figure is trigger time only. The page divides nozzle hours by the nozzle hours you say you get in a day, and that second number is the honest one. Most blasting days deliver far fewer nozzle hours than the shift is long: containment goes up and comes down, staging moves, the pot gets refilled, the enclosure has to clear, and the work gets blown down and inspected. If you enter eight because the shift is eight hours long, the day count will be optimistic by roughly the factor your trigger time falls short.