Inputs
Balloon size: enter whichever you have. Circumference and diameter are measured on the inflated balloon and treated as a sphere. Don't enter the catalog (flat) size: a 60" foil balloon holds far less than a 60" sphere. A tape around the widest part of a four-panel balloon reads a few percent high. Changing the size type converts the current value. Envelope weight is the empty balloon, including the neck seal or tape and the tie. The calibration example comes from a WB3AK flight using a Cymylar 60" balloon. Replace it with your own flight.
Calibration from a past flight (optional)
Foil balloons stretch, so a volume back-calculated from a real flight is more reliable than a catalog figure. "Use calibrated volume: Yes, as is" uses that flight's volume unchanged, which fits only if your balloon is the same size. To allow for your balloon being stretched more or less, enter the flight balloon's measured size (same rules as above): the float factor is its calibrated volume divided by that measured volume, and "Yes, scaled" multiplies your balloon's size by it. Stretch at float also depends on overpressure, which this calculator doesn't model, so the factor carries over best with a similar balloon and free lift.
Float altitude vs free lift
Float altitude (feet)
Overpressure at float below 5%: likely to go slack and sink at night. no fit = the fill gas exceeds the balloon's volume.
Model and assumptions
Float altitude is where air density equals total system mass (envelope + payload + gas) divided by the balloon's full volume. The atmosphere is the 1976 International Standard Atmosphere, valid from 0 to 32 km; real temperatures and pressures vary with season and latitude. Gas mass comes from total lift at launch, assuming pure gas at sea level (1.225 kg/m³, 15 °C). Overpressure at float is free lift divided by total system mass, with the gas at air temperature: sun heating adds several percent and night cooling removes a few. The volume is held fixed, so stretch under higher overpressure isn't modeled.