Apply the Tsiolkovsky rocket equation to find delta-v, mass ratio and propellant mass from specific impulse and vehicle masses.
Ideal rocket equation: it assumes constant specific impulse, no gravity or drag losses and instantaneous burns, so real mission delta-v budgets are larger.
Delta-v (m/s)
How to use this tool
Use seconds: around 300 s for a storable chemical rocket, around 450 s for hydrogen-oxygen, thousands for electric propulsion.
Wet mass includes propellant; dry mass is structure plus payload after burnout.
This is the velocity change the propellant can deliver in free space, before gravity and drag losses.
Delta-v grows only logarithmically with mass ratio, so each extra km/s costs disproportionately more propellant.
Formula & worked example
Isp = 300 s, wet mass 1,000 kg, dry mass 200 kg: delta-v = 300 x 9.80665 x ln(5) = 4,734.96 m/s (4.735 km/s) from 800 kg of propellant.
Limitations — read this before you rely on it
Idealised model: constant Isp, no gravity loss, no drag, no staging or finite-burn effects. Real launch delta-v budgets add roughly 1.5-2 km/s of losses for reaching low Earth orbit.
Frequently asked questions
Each increment of velocity needs propellant that itself must be accelerated, so returns diminish: doubling the mass ratio adds a fixed delta-v, not double.
Thrust per unit weight flow of propellant, in seconds. Higher Isp means more delta-v from the same propellant mass.
Low Earth orbit needs about 9.4 km/s including losses, even though orbital speed itself is about 7.8 km/s.
Yes: dropping empty tanks resets the mass ratio, which is why multi-stage rockets beat single-stage ones for the same payload.
Yes, as long as both masses use the same unit, because only their ratio enters the equation.