How to Accumulate a Result with REDUCE in Excel

REDUCE walks through a list carrying a running value, so you can apply rules step by step and get one final answer.

You will learn to

• Accumulate a running value from a starting point
• Skip steps that break a rule
• Simulate a policy such as automatic reordering

How it works

REDUCE(initial_value, array, LAMBDA(accumulator, value, calculation)) starts with the initial value, then visits each item of the array. The LAMBDA returns the new accumulator each time, and REDUCE returns the final one: =REDUCE(E2,B2:B8,LAMBDA(stock,m,stock+m)) gives closing stock from an opening balance and a list of movements.

The power is that each step can depend on the running total so far. =REDUCE(E2,B2:B8,LAMBDA(s,m,IF(s+m<0,s,s+m))) skips any order that would take stock below zero — something a plain SUM cannot do.

=REDUCE(E2,B2:B8,LAMBDA(stock,m,stock+m))