How to Calculate Straight-Line Depreciation

The monthly charge is right on paper and wrong in the ledger

The formula for straight-line depreciation is simple, which is why it is usually done in a spreadsheet. The trouble starts when real assets meet it: an asset is bought on the 20th, a salvage value is added later, a month is skipped, or one asset is disposed part way through.

Each of those breaks the neat division. The rounded monthly amount times the number of months does not equal the cost, so a few cents are left on the books at the end of the life, or the asset is depreciated below its salvage value.

When the calculation is not stored with the asset, nobody can tell which assumption produced a posted charge, and auditors ask for it.

Why this happens

• The useful life and salvage value are kept in a different file from the asset.
• Rounding is applied to the monthly amount and never corrected at the end.
• The first month is treated differently by different people.
• Assets entered part way through their life are started as if they were new.

Biznsbook addresses this through straight-line depreciation, salvage value, useful life when Accounting and Finance Management are licensed. Each asset carries its own depreciation settings, and the receipt or conversion that brought it in stays linked to the same record.

Books and registers drift when depreciation is worked out in one place and posted in another. Keeping the settings on the asset, reviewing a preview and posting one journal keeps the register, the ledger and the reports telling the same story.

Step-by-step: Calculate Straight-Line Depreciation

Built for accountants, finance controllers and fixed asset managers who depreciate and report on company equipment in Biznsbook. It needs Accounting and Finance Management as well as Fixed Assets Tracking.

1. Work out the depreciable amount. Cost less salvage value. For a 1,200 laptop with a 200 salvage value the depreciable amount is 1,000. If there is no salvage value it is the full cost.
2. Choose the useful life in months. Use the life your policy gives for the class of asset, converted to months. Three years is 36 months, five years is 60.
3. Divide. Depreciable amount divided by months gives the monthly charge. 1,000 over 10 months is 100 a month. Biznsbook recalculates the remaining amount over the remaining months each month, which gives the same result and absorbs rounding.
4. Decide when the first month is. By default the month the asset starts counts as a full month. If your policy starts depreciation the month after, change the first-month convention in Accounting settings.
5. Set it on the asset or its category. In Finance, Asset categories, choose Straight line, the useful life and the salvage percentage. Assets in the category start with those values. You can change them on any asset.
6. Enter existing assets with what is already depreciated. For an asset you already own, tick Existing asset, enter the depreciation taken to date and the date it is stated up to. Biznsbook spreads the rest over the remaining life.
7. Run the month and check the preview. In Depreciation runs the preview shows the charge for each asset. Compare a few against your own calculation before you post.
8. Check the last month. In the final month of its life an asset is written down to its salvage value exactly, so the total charged equals cost less salvage.

Review results after the first full monthly cycle. Adjust roles, mappings, or approvals where the same exception repeats.

Screen names in this guide match the product: Finance, Depreciation runs, Fixed assets, Asset categories, Capitalization queue and Asset reports. Every screen has a Help button.

Common mistakes to avoid

• Mistake 1: The useful life and salvage value are kept in a different file from the asset. Repeating this each month usually shows up first in depreciation that does not match the register, a disposal with the wrong gain or loss, or a roll-forward that has to be rebuilt.
• Mistake 2: Rounding is applied to the monthly amount and never corrected at the end. Repeating this each month usually shows up first in depreciation that does not match the register, a disposal with the wrong gain or loss, or a roll-forward that has to be rebuilt.
• Mistake 3: The first month is treated differently by different people. Repeating this each month usually shows up first in depreciation that does not match the register, a disposal with the wrong gain or loss, or a roll-forward that has to be rebuilt.
• Mistake 4: Assets entered part way through their life are started as if they were new. Repeating this each month usually shows up first in depreciation that does not match the register, a disposal with the wrong gain or loss, or a roll-forward that has to be rebuilt.

Note each recurring gap in the monthly review; each should map to a step in this guide.

Best practices that hold up as you scale

• Work out the depreciable amount — Cost less salvage value.
• Choose the useful life in months — Use the life your policy gives for the class of asset, converted to months.
• Divide — Depreciable amount divided by months gives the monthly charge.
• Decide when the first month is — By default the month the asset starts counts as a full month.
• Set it on the asset or its category — In Finance, Asset categories, choose Straight line, the useful life and the salvage percentage.

Teams that post depreciation every month, post disposals when they happen and clear the capitalization queue at month end find year end a review rather than a rebuild.

How Biznsbook supports this workflow

straight-line depreciation is part of Biznsbook fixed asset accounting. Every asset has one record with its cost, depreciation settings, accumulated depreciation and net book value, and it links back to the receipt or conversion that created it.

salvage value is part of Biznsbook fixed asset accounting. Every asset has one record with its cost, depreciation settings, accumulated depreciation and net book value, and it links back to the receipt or conversion that created it.

useful life is part of Biznsbook fixed asset accounting. Every asset has one record with its cost, depreciation settings, accumulated depreciation and net book value, and it links back to the receipt or conversion that created it.

Running depreciation and posting disposals are separate permissions held by finance roles. Viewing finance data, creating journals and managing tracked assets are also separate.

Suggested implementation timeline

1. Week 1: Agree the depreciation policy with your accountant and set up an asset category for each class of asset.
2. Week 2: Enter existing assets with their depreciation to date and start using straight-line depreciation for the first month.
3. Week 3: Compare the first posted run with your own workings and correct any category defaults.
4. Week 4: Post disposals, clear the capitalization queue and review the register.
5. Ongoing: Monthly review using a worked example and the asset reports.

Related guides

Start with run monthly depreciation, then calculate straight-line depreciation, record asset disposal gain or loss and prepare a fixed asset roll-forward.

Also see receive assets from purchase orders and manage fixed asset depreciation schedules. All guides: asset tracking. Product page: Asset Tracking.

Why remaining amount over remaining months

Dividing cost by life once and multiplying is easy, but it accumulates rounding. Biznsbook divides what is left to depreciate by the months that are left, every month. The charge stays level and the last month closes the gap.

The same rule handles an asset entered part way through its life. With depreciation to date on record, the rest is spread over the remaining months without any special case.

When straight line is not the right choice

Assets that lose value faster early on, such as vehicles or technology, are sometimes depreciated on a declining balance. Biznsbook supports it per category or per asset. Pick the method your accountant uses for the class and keep it consistent.

Tax depreciation follows its own rules and can differ from book depreciation. The tax rates on a category are for reporting and do not change what is posted.

Metrics to track monthly

• Draft depreciation runs not yet posted
• Assets with no depreciation method or useful life
• Assets waiting in the capitalization queue
• Disposals marked Disposal to post
• Difference between register net book value and the ledger

Start with three metrics; trend direction matters more than a single point-in-time snapshot.

Manual handling vs Biznsbook fixed asset accounting

Compare doing this by hand with Biznsbook straight-line depreciation and related capabilities.

A worked example

A machine costs 12,000 with a 2,000 salvage value and a 5 year life, which is 60 months. The depreciable amount is 10,000, so the monthly charge is 166.67. After 12 months about 2,000 has been charged and the net book value is about 10,000. In the last month the charge is whatever brings the book value to exactly 2,000.

Write this into your month end checklist and revisit it when you add asset categories or change your policy.

Frequently asked questions

How this differs by industry