What is Year-over-year growth?
Change versus the same period last year — (current − prior) ÷ prior, the growth measure that cancels seasonality.
Year-over-year (YoY) growth compares a period to the same period one year earlier: this September against last September. The calendar alignment is the point — it cancels seasonality automatically, which comparing to last month (month-over-month) never does. It's the default growth quote in board decks because a year is long enough to smooth noise and short enough to feel current.
The formula is a percentage change with the prior-year period as the denominator. The denominator convention matters for costs too: a cost line growing +36% YoY while revenue grows +13% is a margin quietly eroding — YoY on both sides of the P&L is how you catch it early.
Two cautions. Base effects: against a depressed prior year (a lost quarter, a one-off), YoY growth flatters — read it next to the dollar change. And mix: total-company YoY blends every segment's growth; a 12.6% blended rate can be 14% software and 8% services, with the blend drifting as the faster line grows.
Formula
YoY growth % = (current period − same period prior year) ÷ prior year × 100
Same calendar period, always. For monthly data use the same month; for quarters the same quarter — mixing horizons is how a "growth rate" becomes meaningless.
The Sumwise question
Show September revenue year over year by account: actual vs prior year with $ and % growth, biggest growers first.
Runs on gl_2026.csv. The September GL sample ships actual and prior_year scenarios side by side — exactly the two columns a YoY question needs.
Worked example — gl_2026.csv
| Total revenue: $540,500 vs $480,000 PY | +12.6% |
| Software revenue: $412,000 vs $361,000 PY | +14.1% |
| Services revenue: $128,500 vs $119,000 PY | +8.0% |
| Contractor spend: $66,500 vs $48,800 PY | +36.3% ⚑ |
Revenue is +12.6% YoY ($540,500 vs $480,000) — software +14.1%, services +8.0%. The watch item is contractor spend at +36.3% YoY: growing nearly 3× faster than the revenue it supports.
gl_2026.csv — drop it into the demo's Your-own-CSV tab (or Excel) and re-run the example.
Common questions
YoY or MoM — when do I use which?
YoY as the headline (seasonality cancels); MoM to spot inflections fast (a churn spike, a launch). MoM needs seasonal adjustment or a same-month benchmark to mean anything in a seasonal business.
What about CAGR?
CAGR is the multi-year version — the constant annual rate that connects a start value to an end value over N years. Use YoY for the current trajectory, CAGR for the long-run story; they answer different questions and can disagree for years at a time.
How do I handle a prior year that was zero or negative?
Percent change breaks (division by zero, or a misleading sign flip). Report the dollar change instead, or use a "multiple of" framing — conventions boards accept, formulas that don't explode.
Compute year-over-year growth on your own export, every month
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Related terms
Variance percent
How far an actual landed from its benchmark, as a share of the benchmark — (actual − plan) ÷ plan.
Seasonality
A pattern that repeats on a fixed calendar — measured as a seasonal index, period average ÷ overall average.
ARR
Annual recurring revenue — the yearly value of active recurring contracts, usually MRR × 12.