I'm trying to make a profit and loss report that takes some of my accounts such as "Expenses:Gas" and "Expenses:Automotive" and lets me apply a percentage to them (so that I can include a portion that relates to my business as opposed to personal driving)

for example, lets say I figure out at the end of the year that 30% of my driving miles were business related to that particular business, I would want the report to include 30% of those accounts totals in the profit and loss report as opposed to the full amount of the account.

If there's not a way to do this from within gnucash, how can I access the database with another program to get what I want?

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    This doesn't sound like a very good method to manage your business expenses. Depending on the rules in your country, this may not be an acceptable accounting method at all.
    – littleadv
    Commented Dec 12, 2022 at 23:59
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    it's more for my own personal estimation of how profitable certain endeavors of mine are, i don't plan on using that method for tax purposes Commented Dec 13, 2022 at 0:13
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    @littleadv In the US, that's exactly how the government requires you to do it. Well, you have a choice: Either figure out the business use percentage and multiply all expenses by that percentage, or count how many miles you travelled for business purposes and deduct a standard amount per mile. (That changes every year.)
    – Jay
    Commented Dec 15, 2022 at 3:50
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    @Jay no, not at all. In the US you're required to have a contemporary mileage log, and you cannot claim vehicle expenses without it at all. You then can prorate the expenses based on miles, or use flat rate per mile deduction, sure, but you can't just "figure out" at the end of the year.
    – littleadv
    Commented Dec 15, 2022 at 5:24
  • @littleadv Sorry if I was unclear. I didn't mean that you could just make up a business use percentage. Sure, you have to justify it. By "figure it out at the end of the year" I meant, calculate the business use percentage from a mileage log. You can't do that until the end of the year because you don't know the total number of miles driven until the year is over.
    – Jay
    Commented Dec 15, 2022 at 18:15

3 Answers 3


A couple of things are relevant:

Regardless, this is simple with contra transactions for each account at the end of the year. Let's assume automotive expenses of $1000:

-1000 Expenses:Automotive
  300 Liability:Contra:Automotive:Deductible
  700 Liability:Contra:Automotive:NonDeductible

Include the contra accounts when you run tax reports otherwise exclude them if you don't want tax reports.

You could export the relevant accounts to a spreadsheet, do the calculations and import the transactions as csv: https://www.gnucash.org/docs/v4/C/gnucash-help/trans-import.html#trans-import-csv like Example 6.1. Sample Multi-split.csv


One option is to record the gross expense amounts in GnuCash, then export the values (from a Profit and Loss Report, or a Trial Balance, or whatever report best suits your purpose) to a spreadsheet. In your spreadsheet you can apply the 30% factor to the relevant expense totals.

GnuCash is not really great at exporting a report as a data file, but it can be achieved with a slight workaround. Please see my answer to this question - Exporting Account Totals in GNUCash


I don't use GnuCash and don't know anything about it. But I'd think the simple thing to do is to just calculate the business use percentage and then enter numbers that reflect this percentage. Like if your business use is 30%, and you spend $80 on oil changes, then instead of entering $80 for oil changes, enter 30% of $80 or $24.

The main catch I see to this is that you are presumably entering expenses as they occur, before you know the business use percentage. So you'd have to create some dummy accounts for the real numbers, and then separate accounts for the pro-rated numbers.

Personally, when I had a business where I had a car expense, I kept the actual expenses in a spreadsheet and didn't enter them into my accounting system until after I knew the business use percentage. This was clumsy in principle but easy enough to do in practice.

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