Not so. It depends on which units of measurement you use to calculate the guide number. The units can be anything - meters, feet, cubits, versts... just convert the units and use the conversion factor as a multiplier or divider for the guide number. For instance, if the given guide # is for feet, but you need to know the # for meters, since 1 meter is ~3.33 feet, use that as a divider. Gn 80(ft) equals ( 80 divided by 3.33) gn 24(m). Gn 80(ft) equals (80 times 12) gn 960(inches).