Split a shared monthly bill by what each client actually used

Run software for more than one customer and you end up with bills that cover all of them at once. One flat monthly charge for email. One for hosting. One for the database plan. The bill doesn't say who used what, and it's the same number whether a client sent 200 messages that month or 2,000.

The easy answer is to divide it evenly. That's what I did, and it's simple, but it isn't true. A quiet site pays the same as a busy one, and when you charge a client their share at cost, an even slice means someone is paying for someone else.

So the back office I run Forecheck on now splits these bills by what each one actually used.

What it does

Each shared bill gets divided in proportion to real usage for that billing period. Email is split by how many messages each sender sent. Hosting is split by what each site used. A database plan is split by each database's own charges on the same invoice.

The shares land where they belong. A client's share shows up on that client, ready to bill at cost. A share for one of our own sites goes to overhead. A share for a charity project keeps its charity label. The receipt still adds up to exactly what was charged, down to the cent.

I don't press anything for this. It happens on its own the morning after a billing period ends.

The rules it follows

These bills charge a month ahead, so the real usage for a receipt isn't known until its month is over. That one fact shapes most of the rules:

• A share isn't final until the month is. While a billing period is still running, a client's share of that bill shows on their list but can't be ticked for billing. It might still change, and once something is billed it stays billed.
• Billed stays billed. A receipt already on an invoice is never split again. If I cancel that invoice, the receipt comes back.
• My decisions win. If I set a receipt's shares by hand, the automatic split leaves them alone.
• One receipt, one month. A split applies to its own receipt, so adding a new client this month can't quietly change what last month's receipts said.
• A new client or site gets noticed. If something new shows up with a real share of a bill, the first page of the back office says so, and I place it in one step. Until then its share sits in overhead, where it's visible. Anything under a tenth of a percent stays quiet.

Why I don't review every split

My first instinct was to check each one, every month. Then I looked at what a mistake could actually cost. A wrong share between my own sites moves nothing that matters. A wrong share on a client only matters when it's billed, and billing already goes through me: I see every receipt and every amount, and nothing goes out until I tick it and send it.

So that's where the review lives. The rest of the month it stays out of my way, and it only speaks up when it needs an answer it can't work out on its own.

That's the kind of thing custom software is good for. Off-the-shelf bookkeeping has no idea how your bills map to your customers. Stop bending your business to fit the software.

If you've got a bill like this you've been dividing on a napkin, tell me about it.