Stall Speed at Weight Calculator
Published stall speeds are quoted at maximum gross weight, but most flights are lighter, which lowers the actual stall speed. Because lift varies with the square of speed, stall speed scales with the square root of the weight ratio. Enter the published stall speed and its reference weight, then your actual weight, to get the adjusted stall speed. Every figure is your own input, so nothing is assumed about a specific aircraft.
Stall speed at weight formula
Adjusted Vs = published Vs * sqrt(actual weight / reference weight)
Change (knots) = adjusted Vs - published Vs
Change (percent) = (adjusted Vs / published Vs - 1) * 100
The same units must be used for both weights. The result is valid for the same configuration (flap setting) and air density at which the published stall speed applies.
Worked example
- Published Vs = 50 knots at 2,550 lb gross.
- Actual weight 2,100 lb gives a weight ratio of 0.8235.
- sqrt(0.8235) = 0.9075, so adjusted Vs = 50 * 0.9075 = 45.37 knots.
- That is about 4.63 knots, or 9.25 percent, lower than the published figure.
Stall speed: frequently asked questions
How does stall speed change with weight?
Stall speed scales with the square root of the weight ratio. New stall speed = published stall speed * sqrt(new weight / reference weight). A 20 percent reduction in weight lowers stall speed by about 11 percent, because sqrt(0.8) is about 0.894.
Why use the square root of weight?
At the stall, lift equals weight and the lift coefficient is at its maximum. Lift is proportional to the square of speed, so to balance a change in weight the speed changes with the square root of the weight ratio. This relationship is exact for the same configuration and air density.
Where do I find the published stall speed?
The published stall speed at maximum gross weight (Vs or Vs0/Vs1) appears in the aircraft Pilot's Operating Handbook and on the airspeed indicator markings. Enter that value and its reference weight; both are user inputs so no figure is assumed.
Does this account for bank angle or flaps?
This tool adjusts only for weight. A turn raises stall speed by the square root of the load factor, and flap setting changes the maximum lift coefficient, which changes the baseline stall speed. Use the published Vs for the configuration you are flying.
Official sources
- FAA: Airplane Flying Handbook (stalls and airspeed).
- FAA: Pilot's Handbook of Aeronautical Knowledge (lift and stall).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.