Splitwise's own account of its founding: it started with Jon's rent-splitting calculator, a simple fairness tool for dividing rent between roommates. The team only built the general Splitwise app afterward, once they noticed everyone they knew was already improvising ways to track shared debts (fridge notes, spreadsheets) and needed something better.

“It started with Jon's rent-splitting calculator”

Neither source states directly how the calculator's first users found it, but the pre-launch validation survey was drawn from Jon's own social circle (his Facebook feed and gchat-status audience), which is the clearest signal available for how the earliest usage was seeded.