Definitions
Isp = F / (ṁ g0) = ve / g0
ve = Isp × g0
F = ṁ ve = ṁ × Isp × g0
g0 is the standard gravity 9.80665 m/s² used to define specific impulse, even for a vacuum burn — the same convention as the rocket equation page. Isp is reported in seconds. In US customary units the same number of seconds is F (lbf) / ẇ (lbf/s). A 10,000 N engine at 300 s therefore consumes about 3.399 kg/s of propellant, with ve = 2,942 m/s.
Ideal Δv
If you enter a mass ratio, the page also reports the ideal Tsiolkovsky increment Δv = ve ln(m0/mf). Multi-stage problems, mixed units of mass, and a payload breakdown belong on the rocket-equation calculator.
Sources
Sutton & Biblarz, Rocket Propulsion Elements; NASA Glenn Beginner’s Guide to Propulsion (specific impulse); Tsiolkovsky rocket equation.
Frequently asked questions
Why is Isp in seconds, not meters per second?
The modern definition divides exhaust velocity by standard gravity, leaving a time. The useful physical quantity is still ve = Isp × g0. Some SI texts call ve itself a specific impulse in N·s/kg (which is m/s); this page keeps the seconds convention used in the rocket-equation calculator.
Does 1 lbf per 1 lbf/s really equal 1 second of Isp?
Yes. In US customary units Isp (seconds) is thrust in lbf divided by weight flow in lbf/s, so 1 / 1 = 1 s. The same identity holds in SI once g0 is included: F = ṁ g0 Isp.
Sea-level or vacuum Isp?
They differ because of nozzle back-pressure. Enter the Isp (or ve) that matches the condition you mean. This page does not model a nozzle or an ambient pressure.