Throw scales with flow, and that is the whole trick
A free jet leaving an outlet carries momentum. Its centreline velocity at a distance falls off with that distance, and the constant in front depends on the discharge area and the discharge velocity — which multiply together to give the flow. Hold the outlet fixed and the throw to any chosen end velocity is proportional to the flow through it and inversely proportional to that end velocity. Two multiplications, no curve fitting.
Work an example. A catalogue lists 12 ft of throw at 150 CFM to a 50 fpm terminal velocity. The outlet in the ceiling is measuring 110 CFM because it shares a branch. The throw is 12 times 110 over 150, which is 8.8 ft. In a 16 ft room that jet gives out 7.2 ft short of the far wall — barely over half way across — and no amount of adjusting the blades changes the flow that is arriving.
Three throws for one diffuser
Catalogues publish throw to a stated terminal velocity, and the choice of that velocity changes the number enormously. The same 12 ft at 50 fpm is 6 ft at 100 fpm and 4 ft at 150. All three are correct and all three describe the same jet; they are just following it out to different points. Comparing two products where one is published to 50 and the other to 150 makes the second look like a toy, and the mistake is easy to make because the terminal velocity is usually a column heading rather than part of the number.
The same applies to the flow. A throw quoted without its flow beside it is unusable, and every catalogue prints them together for that reason.
Where the model runs out
It assumes the outlet is unchanged. Change the deflection, the pattern or the core and the discharge area moves, which moves the constant the arithmetic is built on. It also assumes an isothermal jet — air at room temperature. Real supply air is not. A cold jet is heavier than the room and falls out of the ceiling early; a hot jet is lighter and clings, running past the isothermal distance and sometimes staying up at the ceiling entirely, which is the mechanism behind a room that heats badly from a high outlet. Catalogues publish isothermal throws precisely because the correction depends on the temperature difference, and where the supply is far from room temperature the catalogue correction belongs in front of this.
And the split between outlets here is even, which real branch ducts are not. Two outlets on one run with different fitting counts and a damper somewhere are not moving the same air, and if the balance matters, a flow hood settles it in ten minutes and this page then works from measurements rather than from a division.
No target, on purpose
There is no line here saying the throw should reach 75 or 80 percent of the way across the room, or that opposing jets should meet. Those numbers exist, they differ between the people who publish them, and they depend on the ceiling height, the glazing on the outside wall and where people actually sit. The page prints the throw and the ratio to the distance you entered so you can compare them to whatever the design was drawn to. It does not have an opinion about that design.
Questions people ask
How do I scale a catalogue diffuser throw to my actual airflow?
Multiply by the ratio of the flows. For a fixed outlet the throw is proportional to the flow through it, so a catalogue 12 ft at 150 CFM becomes 8.8 ft at 110 CFM. If you also want a different terminal velocity than the catalogue used, divide by the ratio of those — the same jet followed out to a lower end velocity has a longer throw.
What is terminal velocity in a throw rating?
The air speed at which the catalogue stops following the jet and calls the throw finished. Common published values are 50, 100 and 150 fpm, and the same diffuser has three different throws depending on which is used — 12 ft to 50 fpm is 6 ft to 100 and 4 ft to 150. Two products quoted to different terminal velocities cannot be compared until they are put on the same one.
Does a cold supply jet throw as far as the catalogue says?
No. Catalogue throws are published isothermal, meaning the jet is at room temperature. Cooler supply air is denser than the room and drops out of the jet before it reaches the published distance; warmer air is lighter and stays up at the ceiling past it. Where the supply is well away from room temperature, the catalogue usually carries a correction and it should be applied before this arithmetic.
What throw should I be aiming for?
This page does not say, and that is deliberate. Published guidance on how far a jet should reach differs, and the answer depends on the ceiling height, the load on the outside wall, whether the outlets oppose each other and where people sit. The page gives the throw and its ratio to the distance you entered so you can put it beside whatever the design was worked to.
Do two outlets in a room each throw half as far?
If they split the flow evenly, yes — throw is proportional to the flow through the outlet, so half the flow is exactly half the throw. Real branches rarely split evenly, though. Different fitting counts and a partly closed damper put a substantial difference between two outlets that look identical from the room, and a flow hood is the only way to find out which is which.