The Excel REDUCE function walks through an array one element at a time, carrying a running accumulator, and returns the single final value. It is Excel’s loop: anything shaped like “start with X, then for each value do Y” is a REDUCE. Where MAP gives back a same-size array, REDUCE folds everything down to one answer — and its sibling SCAN shows every intermediate step along the way.
acc starts at 0; for each value v the LAMBDA returns the new accumulator. After the last element, REDUCE returns the final acc.
Syntax
| Argument | Description | |
|---|---|---|
initial_value | Optional | The accumulator’s starting value. Use 0 for sums, 1 for products, "" for building text. Technically optional, but omitting it makes the start value blank — set it explicitly. |
array | Required | The range or array to fold, processed left-to-right, top-to-bottom. |
lambda | Required | A two-parameter LAMBDA: the running accumulator, then the current value. Whatever the body returns becomes the accumulator for the next element. |
Available in: Excel for Microsoft 365 and Excel for the web only. Older versions show #NAME?. Parameter order matters: accumulator first, value second.
Fold a list to one value
Step counts sit in B2:B6, but for the challenge only the first 10,000 steps per day count. No single built-in aggregate does “cap, then sum” — REDUCE does:
| A | B | C | |
|---|---|---|---|
| 1 | Day | Steps | Capped at 10,000 |
| 2 | Mon | 8,400 | 46,600 |
| 3 | Tue | 12,750 | |
| 4 | Wed | 9,100 | |
| 5 | Thu | 14,300 | |
| 6 | Fri | 9,100 |
Trace it: acc starts at 0 → +8,400 → +10,000 (capped) → +9,100 → +10,000 (capped) → +9,100 = 46,600. Each pass, the LAMBDA’s return value becomes the next acc.
The accumulator doesn’t have to be a number — build text by appending:
Try it: watch the accumulator fold
Pick an operation, then click Step to feed the array into the LAMBDA one value at a time and watch the accumulator change.
REDUCE vs SCAN, and real-world folds
SCAN takes exactly the same arguments but returns every intermediate accumulator instead of just the last one. Same LAMBDA, different output shape:
Use REDUCE when only the destination matters, SCAN when the journey is the report.
A favorite real-world fold — apply a whole column of percentage discounts in sequence to one starting price:
REDUCE can even accumulate an array: start with a header row and VSTACK qualifying rows onto it as you go. That pattern — REDUCE + VSTACK — is how pros build filtered, reshaped tables that no single function can produce.
Don’t over-REDUCE: plain totals are still SUM’s job. Reach for REDUCE when the step logic is custom — caps, conditions, compounding, or building something up element by element.
Errors & common pitfalls
#CALC! — the LAMBDA doesn’t declare two parameters. REDUCE always passes the accumulator and the current value. LAMBDA(v, …) with one parameter fails; you need LAMBDA(acc, v, …).
#NAME? — older Excel. REDUCE exists only in Excel for Microsoft 365 and Excel for the web. Excel 2021 and earlier don’t recognize it.
Pitfall: parameters swapped. The accumulator comes first. LAMBDA(v, acc, acc+v) still calculates — with the meanings reversed — and for non-symmetric logic like subtraction or text-building it silently produces wrong answers.
Pitfall: wrong initial_value. Starting a product at 0 returns 0 forever; starting a text-builder at 0 gives you a stray leading zero. Match the start value to the operation: 0 for sums, 1 for products, "" for text.
Pitfall: expecting a spilled column. REDUCE returns one value (unless you deliberately accumulate an array). If you wanted the running totals down the column, you wanted SCAN.
Practice workbook
Frequently asked questions
What does the Excel REDUCE function do?
What's the difference between REDUCE and SCAN?
What's the parameter order in REDUCE's LAMBDA?
LAMBDA(acc, v, body). Swapping them doesn't raise an error — the calculation just runs with the meanings reversed, which corrupts any non-symmetric logic.When should I use REDUCE instead of SUM or SUMPRODUCT?
Why does REDUCE return #CALC!?
Which Excel versions support REDUCE?
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