Choosing a recovery parachute size
How canopy diameter, rocket mass and drag coefficient set your landing speed, and a simple method to size a chute for a soft, driftable recovery.
Landing speed is the real target
The point of a recovery parachute is not just to slow the rocket but to land it slowly enough that nothing breaks. Descent rate depends on how much weight the canopy carries and how much drag it makes at a given size. A tiny chute under a heavy rocket falls fast, while an oversized chute lands feather-soft but drifts far downwind. Sizing is the balance between a gentle touchdown and keeping the rocket on the field, and this calculator lets you test both ends before you cut fabric.
Why diameter matters more than it looks
Canopy area, not diameter, sets the drag, and area rises with the square of the diameter. That means a modest increase in diameter buys a large increase in area and a real cut in landing speed. Going from a 0.45 m to a 0.6 m chute, for example, is only a third more across yet nearly doubles the area. Because descent rate falls with the square root of area, that upgrade drops the speed by a noticeable margin, which is often the quickest way to turn a hard landing into a safe one.
Reading drag coefficient off the canopy shape
The drag coefficient captures how efficiently a canopy catches air. Flat, single-sheet parachutes billow into a shallow dome and tend to have a high coefficient, often near 1.5, while true hemispherical canopies sit closer to 0.75. Spill holes, which vent air from the centre to reduce swinging, trade a little drag for stability. Using your canopy's real coefficient instead of a generic guess is what keeps the predicted landing speed honest, so check the kit instructions or the maker's specification.
Trading softness against drift
A slower descent is safer on landing but spends more time aloft, so the rocket drifts farther in any breeze. On a small field a slightly firmer landing under a smaller chute can be the better call, because a soft landing three fields away is not really a success. Many fliers pick the smallest canopy that still keeps the descent rate under about 5 m/s, then use a streamer or a smaller drogue at apogee with a main chute deployed lower down. Testing a few sizes in the calculator makes that trade-off concrete before launch day.