Use the footprint of the roof area draining to the gutter you're collecting from, not the full roof if you're only using one section.
What You're Actually Collecting
The math starts simple: 1 inch of rain on 1 square foot of roof equals 0.623 gallons. That's not a rule of thumb, it's exact — a square foot of area times one inch of depth is 1/12 of a cubic foot, and a cubic foot of water is 7.48 gallons. Multiply your roof area by that number and you get the theoretical maximum.
Real collection is always less than the theoretical max because of splash-out, evaporation, gutter overflow during heavy storms, and any first-flush diverter sending the initial dirty runoff away from your storage. 85% efficiency is a reasonable working number for a typical home setup.
A standard 55-gallon rain barrel fills fast. Even a modest 20x30 roof section collects around 370 gallons per inch of rain at 85% efficiency, which means one good New England thunderstorm can overflow a single barrel many times over. If you're serious about storage, look at a larger cistern or link multiple barrels together.
Skip the collected water for drinking without treatment, and think twice about using it directly on edible crops if your roof is asphalt shingle, since runoff can carry small amounts of the same chemicals that give shingles their weatherproofing. It's excellent for ornamental beds, lawns, and non-edible landscaping regardless of roof type.
Massachusetts, New Hampshire, and the rest of New England place no real restrictions on rainwater collection, unlike some western states with water rights laws — but always check your specific town if you're planning a large cistern, since some local building codes have requirements for larger installations.