The bigger issue was not even the math. It was that shared expenses are rarely shared equally. Maybe only a few people joined for dinner. Maybe one car paid for parking. Maybe the kids’ snacks should not be split across everyone. Most tools I tried felt either too rigid, unnecessarily complicated, or locked behind paid versions.

So I either had to come up with a more sophisticated spreadsheet with formulas and whatnot and keep improving it every time a new scenario emerged, or take the opportunity to build a small tool/app for myself called CostShare. Thanks to AI for some help along the way. I am not a software engineer by trade, and this is intentionally just a lightweight frontend app with no backend, accounts, or cloud infrastructure behind it. I simply wanted something practical that worked well. With some help from AI along the way, it took me a couple of days to put together.

It is intentionally simple. Add people, add expenses, choose who shared each cost, and it calculates balances and settlements automatically. It also works well on mobile, exports spreadsheets, supports printable summaries, and lets you save or restore sessions later.

I tested it using the kinds of trips and group situations I have experienced myself, and I know I will keep coming back to it whenever costs are shared between people. If it ends up being useful to others too, even better.

You can try it here: https://costshare.alestead.com

None of this is a big deal of course. People have managed shared expenses with spreadsheets, notes apps, calculators, and group chats forever, and probably will continue to do so. This simply made the process a little smoother for me. There is something oddly satisfying about removing even a tiny bit of friction from small real-life situations, and honestly just building something practical that I know I will personally use felt like a good little accomplishment.