Some invoices belong to one account and some don't. A phone bill covering three departments, a builder's merchant invoice with materials for two jobs, an insurance premium spanning two financial years — each needs the cost broken across codes rather than dropped into one.
The mechanics are easy. Choosing a basis you can defend, and not redoing it by hand every month, is the part worth thinking about.
When to split, and when not to
Splitting has a cost: it takes longer to enter and longer to check. It's worth it when the split changes a decision.
Worth splitting:
- Costs a budget holder is accountable for, where the wrong code makes their budget wrong
- Anything recharged to a client, a project or another entity
- Costs spanning a period end, where the timing affects the accounts
- Materials across jobs, where job profitability is a number you actually use
Not worth splitting:
- Small amounts. A £40 invoice split three ways is three coding decisions and a rounding argument for £13 of information.
- Anything where you'd never look at the components separately
- Splits that would be an even division across everything — that's what an overhead allocation in reporting is for, not a coding decision per invoice
A working threshold: if the smallest slice wouldn't change anyone's decision, don't create it.
Pick a basis and write it down
The split has to rest on something. The usual bases, roughly in order of how easy they are to defend:
Actual usage. The invoice itself shows the breakdown — line items per department, metered usage per site. Strongest basis, because it isn't an estimate, and it depends entirely on your capture reading the lines rather than the header.
A physical measure. Floor area for rent and rates, headcount for per-person services, vehicle count for fleet insurance. Defensible and stable.
Time. For anything spanning a period — an annual premium split across twelve months, a service covering two financial years.
Revenue or activity share. Weaker, because it moves. Fine for genuine overheads, poor for anything a budget holder is accountable for, since their allocation changes for reasons unrelated to them.
A fixed percentage agreed once. Pragmatic, and the most common in practice. Reasonable as long as it's reviewed occasionally and someone can say where it came from.
The important part isn't which you pick. It's that it's written down somewhere, so the next person doesn't invent a different one and the auditor doesn't have to ask.
Doing it in practice
Most accounting packages take multi-line bills where each line carries its own account, tax rate and tracking. The practical guidance:
One line per coding decision, not per invoice line. An invoice with twenty items that all code the same way is one line. An invoice with twenty items across three departments is three.
Split at the level you report at. If you report by department or cost centre, split by that. If you also need it by cost type, that's the account code doing that job — don't try to encode two dimensions in one field.
Use tracking or dimensions rather than multiplying accounts. "Travel" tracked by department in Xero is far better than separate travel accounts per department. Same reporting, fraction of the chart of accounts.
Watch the rounding. Three-way splits of odd amounts leave a penny. Put it on the largest slice by convention, and make that the convention so nobody re-derives it.
Keep the tax treatment per line. A split invoice can have different VAT treatment on different lines, and assuming one rate across the whole thing is a common silent error.
Stop doing the same split by hand
The recurring ones are where the time goes. A phone bill split the same way every month for three years is thirty-six identical coding decisions.
Repeating bill templates. If the supplier, amount and split are stable, a template holds the whole thing and you're approving rather than entering.
Supplier defaults with a standard split, where your package supports it.
Ask the supplier to itemise. Underrated. Many will break an invoice down by site, department or cost centre if asked, and then the split is on the document rather than in your head. This turns a monthly judgment into a monthly transcription — and it means the basis is the supplier's actual usage data rather than your estimate.
Separate accounts with the supplier. For large recurring spend across departments, some suppliers will bill separately per cost centre. That removes the split entirely.
The check worth running
Splits go stale. The percentages were right when they were set and the business has moved.
Once a year, take the recurring splits and ask whether the basis still holds. Headcount has changed, a site has closed, the project finished. A split that was accurate in 2024 and hasn't been touched since is quietly misallocating cost every month, and it's the kind of error that never surfaces on its own because nothing about it looks wrong.
Five minutes per recurring split, once a year, and it stays defensible.
Cribble reads invoices to the line item rather than just the header, so a multi-line invoice arrives with its detail intact and ready to allocate. Coding you apply is remembered per supplier, so a split you set once stops being a monthly decision.