Model Rocket Parachute Size Calculator
Enter the rocket mass, target landing speed, launch-site air conditions, and parachute details. The calculator solves for the needed round-canopy diameter, rounds up to a common size, and shows the likely descent speed, energy, and drift.
Recommended standard diameter (in)30
- Calculated air density (kg/m^3)
- 1.22
- Required effective canopy area (ft^2)
- 3.46
- Minimum outer diameter (in)
- 25.2
- Predicted descent rate with recommended size (ft/s)
- 15.12
- Landing kinetic energy (J)
- 4.82
- Landing kinetic energy (ft-lb)
- 3.55
- Estimated descent time (s)
- 33.07
- Estimated downwind drift (ft)
- 242.53
- Existing parachute assessment
- 24 in canopy: 18.9 ft/s, 7.5 J (5.5 ft-lb); is faster than the target.
Assumes a fully inflated round canopy, steady vertical terminal descent, constant wind, and no horizontal speed at deployment.
How to use this calculator
- Enter the complete descending mass of the rocket after ejection.
- Choose a target descent rate for the rocket and landing surface.
- Enter the field elevation, expected air temperature, drag coefficient, and any spill-hole diameter.
- Add deployment height and wind speed to estimate time under canopy and downwind drift.
- Enter a parachute you already own to compare its predicted descent rate with the target.
How the parachute size is calculated
The calculator uses the drag equation in reverse. At terminal velocity, drag equals the rocket's weight, so the area required for a target descent speed is:
A = 2mg / (rho * Cd * v^2)
- m is the recovery mass converted to kilograms.
- g is standard gravity.
- rho is air density at the entered elevation and temperature.
- Cd is the parachute drag coefficient.
- v is the target terminal velocity.
Air density is estimated from the standard troposphere pressure equation below 11,000 meters, then adjusted with the entered air temperature: rho = p / (Rd * T). Higher fields and warmer air both lower density, which means the canopy has to be larger to make the same drag.
For a round parachute, the effective planform area is the outer disk area minus the spill-hole area:
A = pi * (D^2 - dh^2) / 4
The calculator solves that expression for the minimum outer diameter, then rounds up to the next common nominal size from 6 to 120 inches. It then recalculates the descent rate with that actual standard size, because rounding up usually makes the rocket descend more slowly than the target.
What moves the answer most
Mass, descent rate, and drag coefficient matter most. Doubling mass does not double diameter, but it does require about 41% more diameter when the other inputs stay fixed. Lowering the target descent rate has a stronger effect because velocity is squared in the drag equation. A conservative Cd makes the recommendation larger; a manufacturer-tested Cd can make the estimate more specific.
Spill holes also matter. A small center hole can improve stability, but its open area does not produce drag in this simple model, so the outer diameter must grow to replace that lost area.
What this calculator leaves out
The result assumes a fully inflated parachute descending straight down at steady terminal velocity. It does not model deployment shock, opening time, shroud-line strength, canopy porosity, oscillation, packed volume, ejection reliability, thermals, layered winds, terrain, or horizontal momentum at deployment. Use it as a sizing check alongside ground testing, a flight simulator, manufacturer data, and the applicable model rocketry safety code.
Worked example
For a 16 oz rocket at sea level on a 59 deg F day, with a target descent rate of 18 ft/s, Cd 0.75, and no spill hole, the required effective area is 3.46 ft^2. That corresponds to a minimum round-canopy diameter of 25.2 inches.
The next common size in the calculator's list is 30 inches. With that canopy, the predicted descent rate is 15.1 ft/s and the landing energy is about 4.8 J, or 3.6 ft-lb. From a 500 ft deployment height in a 5 mph average wind, the estimated time under canopy is 33.1 seconds and the simple drift estimate is about 243 ft.
Common questions
What descent rate is safe for a model rocket?
Many sport rockets are sized somewhere around 15 to 20 ft/s, but the right value depends on mass, fin strength, surface hardness, and how much drift the field can tolerate. A fragile scale model may need a slower descent, while a rugged lightweight rocket on grass can often land faster.
How do I weigh a rocket for parachute sizing?
Use the complete descending mass, not the empty airframe weight. Include the spent motor casing, shock cord, parachute, wadding or protector, payload, and any hardware that comes down under the main parachute.
What drag coefficient should I use for an untested parachute?
Use manufacturer test data when it exists. If you do not have a measured value, 0.75 is a conservative starting point for a simple flat round canopy, but real values change with fabric, porosity, spill holes, shroud lines, and inflation shape.
Does a spill hole make a parachute descend faster?
Yes, all else equal. The spill hole removes area from the disk, so the same outer diameter produces less drag area and a higher terminal velocity. The calculator adds that missing area back when it solves for the required outer diameter.
Should I round up to the next parachute size?
Usually yes, if field drift and packed volume allow it. Rounding down makes the rocket land faster than the selected target, while rounding up gives a margin for lower actual Cd, canopy motion, and imperfect inflation.
Why can simulator results differ from this estimate?
Flight simulators may use different drag coefficients, deployment timing, wind layers, reefing or opening behavior, and rocket horizontal velocity at deployment. This calculator is a steady terminal-descent estimate, so it is best used as a cross-check rather than a full recovery simulation.