Diameter is linear, air is not
A blast nozzle wears from the inside out and it wears evenly, so the only thing that changes is the bore. Air demand follows the throat area, and area follows the square of the diameter. That single fact is the whole page.
Work the default through. A nozzle that started at 0.375 in and now gauges 0.4375 in has grown 16.7 percent in diameter, which sounds like something you could ignore. Square it and the throat area is 36.1 percent larger, so the nozzle is pulling 36.1 percent more air. At 100 psi and a flow coefficient of 0.8 that is 254 cfm where the new bore wanted 187, and the extra 67 cfm has to come from somewhere. On a 375 cfm machine the nozzle went from using half the compressor to using two thirds of it, for no extra cleaning at all.
The reason this matters more than it sounds is that the pressure does not stay put. If the machine cannot supply the demand, the nozzle pressure falls until supply and demand meet, and the cleaning rate falls with it. So the wear does not announce itself as an air problem. It announces itself as a job that is taking longer than the last one, which people blame on the abrasive or on the coating.
The two routes to the number
If you have an air table for the nozzle you are using, the rated route is the honest one: take the figure at its quoted pressure and this page scales it with absolute pressure and with the wear ratio. Absolute pressure is the right scaling because a choked nozzle passes mass in proportion to the upstream absolute pressure, so 100 psi gauge is 114.7 psia and 80 psi gauge is 94.7, a ratio of 0.826 rather than the 0.8 you get from the gauge readings alone.
If you do not have a table, the bore route works it out from compressible flow through an orifice. That gives an ideal figure which is always higher than reality, and the flow coefficient brings it back down. The coefficient is doing a lot of work: it absorbs the shape of the throat, the hose, the couplings and the pressure already lost upstream. Treat the default as a placeholder until you can calibrate it against one real measurement.
What a gauge in the hose tells you
The single most useful instrument in blasting is a needle pressure gauge pushed into the hose just behind the nozzle. It answers a question no calculator can: what the air is actually doing at the point of work, after everything upstream has taken its share. A machine set to 125 psi that reads 85 at the nozzle is telling you about hose diameter, hose length and coupling count, and no amount of turning the regulator up will fix a hose that is too small for the flow.
That measurement is also what the nozzle bore has to be read against. A new nozzle on a system already short of air and a worn nozzle on an oversized system can produce the same pressure reading for completely different reasons, and the only way to tell them apart is to gauge the bore.
Where this sits
The abrasive side of the same job is the blast media calculator, which works from the metering valve rather than the nozzle. For shop air rather than blast air, the air compressor CFM calculator adds tools up at their duty cycles and works out how long a tank holds pressure between cut-in and cut-out. If what you are sizing is the extraction rather than the supply, the dust collection calculator covers duct resistance, and the CFM and air changes calculator covers a room.
Questions people ask
How much wear is worth replacing a nozzle over?
That is a cost comparison, and this page gives you one half of it. A bore that has grown a sixteenth of an inch from three eighths is pulling 36 percent more air, and you can price that against the machine you are running: a bigger machine, more fuel, or a longer job at reduced pressure. The other half is what the replacement costs and how long the new one lasts on your abrasive, which only your own records answer. What is worth avoiding is the common pattern of running a nozzle until it visibly rattles, because the air penalty arrives long before anything looks wrong.
Why does the pressure figure use 14.7 added to it?
Because a choked nozzle passes mass in proportion to the absolute pressure upstream of it, and a gauge reads the difference from atmosphere rather than the absolute value. At 100 psi gauge the absolute pressure is 114.7 psia; at 50 psi gauge it is 64.7. Notice that halving the gauge reading did not halve the flow, it took it to 56 percent, because the atmosphere contributes to both. Anyone dividing gauge readings to scale flow is making a small error at high pressure and a large one at low pressure.
Where does the flow coefficient come from?
From you, ideally. Perfectly choked flow through an ideal orifice is a calculable upper bound and every real nozzle passes less, so the coefficient is the ratio between what actually happens and that bound. It absorbs the convergent-divergent shape of the throat, the losses in the hose and fittings, and any pressure already lost before the air reaches the nozzle. The default is a starting point, not a specification. If you have a published air figure for the nozzle you own, divide it by the ideal figure this page prints at the same pressure and use the result: that is your setup measured rather than assumed.
Does a bigger nozzle clean faster in proportion to the air?
Not in proportion, no, and this is worth being careful about. A larger bore moves more abrasive at the same velocity so the blast pattern is wider and more work gets done per pass, but it only does that if the compressor holds the pressure up. Put a larger nozzle on a machine that was already at its limit and the pressure falls, the abrasive slows, and you can end up cleaning less than you were with the smaller one while burning more fuel. The pairing of nozzle to machine is the decision; the nozzle alone is not.
Can I use this for a paint spray gun or an air tool?
No. The arithmetic here assumes choked flow through a fixed round orifice, which is a good description of a blast nozzle at blasting pressure and a poor one for almost anything else. A spray gun has an air cap with multiple ports and runs at pressures where the flow may not be choked at all. An air tool consumes air in a duty cycle rather than continuously, which is a completely different sizing problem and the reason the air compressor CFM calculator on this site works from tool ratings and duty cycles instead.