If you run QuickBooks Online on Essentials or Plus, there is no button that calculates depreciation for you. That feature comes only with the Advanced plan. Intuit's own support article says so: "QuickBooks Online doesn't automatically depreciate fixed assets. Instead, you need to manually track depreciation using journal entries" (Intuit, "Depreciate assets in QuickBooks Online").
Where fixed asset tracking lives in the QuickBooks Online tiers
QuickBooks Online is sold in tiers: Simple Start, Essentials, Plus, and Advanced, each adding capability on top of the last (Intuit, "Account plan comparison"). Essentials and Plus cover the core bookkeeping most small businesses need: invoicing, bills, bank feeds, and on Plus, inventory, project tracking, and class or location tracking. Neither tier includes a fixed asset register or a depreciation engine.
The fixed assets feature is an Advanced-only addition. Intuit's help article on the feature describes what it does once you are on that tier: you enter an asset's purchase date, cost, useful life, and depreciation method, and "the app automatically creates the depreciation schedule," posting entries "on the 1st day of each month" (Intuit, "Add and manage fixed assets in QuickBooks Online Advanced"). It supports straight-line and an accelerated declining-balance method, lets you enter an asset that already has accumulated depreciation from a prior system, and produces reports grouped by asset or asset type.
That leaves a gap for an Essentials or Plus company with a fixed asset register but no other reason to move to Advanced, which also raises the user limit, chart-of-accounts limit, and adds features (role-based permissions, batch operations) a smaller company may not need. Upgrading a whole subscription tier to get one feature is a cost.
What Essentials and Plus users do today
Absent the Advanced feature, depreciation in QuickBooks Online is a manual journal entry, made on whatever cadence the company closes its books (usually monthly). Intuit's own instructions for this are short: create a journal entry, debit the depreciation expense account, credit accumulated depreciation, save (Intuit, "Depreciate assets in QuickBooks Online," cited above). QuickBooks will not tell you if the amount is right; it posts whatever number you type. The calculation, and running it the same way every period, is on you.
In practice this means someone (often the controller, sometimes a bookkeeper who inherited the file) maintains a schedule outside QuickBooks Online, usually a spreadsheet: one row per asset, cost, in-service date, useful life, method, and a column per month. Every close, that spreadsheet produces one number per depreciation expense account, and someone keys a journal entry for it. This works until the spreadsheet's author leaves, a formula gets dragged wrong, or an asset is disposed of and nobody remembers to stop depreciating it.
A worked monthly entry
Sample Company buys a piece of shop equipment on January 1 for $24,000. It has no salvage value and a five-year useful life. Under straight-line depreciation, annual depreciation is cost divided by useful life:
$24,000 / 5 years = $4,800 per year
Monthly depreciation is that annual figure divided by twelve:
$4,800 / 12 = $400 per month
Each month, Sample Company's controller posts a journal entry: debit "Depreciation expense, equipment" for $400, credit "Accumulated depreciation, equipment" for $400. After twelve months, accumulated depreciation on that asset is $4,800, matching the annual figure, and the asset's net book value on the balance sheet (cost minus accumulated depreciation) has dropped from $24,000 to $19,200. The arithmetic is simple for one asset. It becomes tedious and error-prone as the count grows, and once assets are added, disposed of, or transferred mid-year the monthly figure changes partway through.
Book depreciation is not tax depreciation
The entry above is book depreciation, the figure that goes on the financial statements. It is calculated differently from tax depreciation, which for most property placed in service after 1986 follows the Modified Accelerated Cost Recovery System, and can be accelerated further with the Section 179 deduction or the bonus depreciation allowance (Internal Revenue Service, Publication 946, How To Depreciate Property). A company usually needs both figures: book depreciation for the general ledger and the audited financial statements, tax depreciation for the return. QuickBooks Online, with or without Advanced, only produces the book-side journal entry you give it. Confirm the tax-side calculation and elections with your tax preparer; that is a separate schedule, not something to derive from the general ledger entry.
The three practical paths
For an Essentials or Plus company with a fixed asset count of more than a handful (or any mix of methods and useful lives), there are three ways to handle this:
Keep the spreadsheet, keep posting manually. No new cost, but no controls either. Nothing locks a closed month, nothing stops someone from editing January's numbers in June, and nothing produces an audit trail showing who changed what. For a five-asset company that may be acceptable. For fifty assets across several locations, expect an auditor to ask how the schedule is protected from later edits, and have an answer.
Upgrade to QuickBooks Online Advanced. This gets you the built-in fixed asset feature and its automatic monthly posting, but also means paying for the full Advanced tier, including capabilities (higher user limits, role permissions, unlimited chart of accounts) the company may not need yet. It is a reasonable choice if those other Advanced features are also on the roadmap.
Keep the register outside QuickBooks Online and post the entry from there. This is the spreadsheet approach with a register in place of the spreadsheet: locked periods, an audit trail of who changed an asset and when, and support for more than straight-line if the company needs declining balance, sum-of-years-digits, or units of production. Steda is one: a fixed asset register with those depreciation methods, a period close with locked months, and journal entries posted to QuickBooks Online.
Whichever path you choose, an auditor will ask how the monthly number was calculated and whether it can be reproduced. Choose the path that lets you answer.