Index Number Calculator Formula

Understand the math behind the index number calculator. Each variable explained with a worked example.

Use the Index Number Calculator

Formulas Used

Index Number

index_number = (current / base) * base_index

Change from Base (%)

pct_change = ((current / base) - 1) * 100

Current / Base Ratio

ratio = current / base

Variables

VariableDescriptionDefault
currentCurrent Period Value125
baseBase Period Value100
base_indexBase Index (usually 100)100

How It Works

How to Calculate an Index Number

Formula

Index = (Current Value / Base Value) * Base Index

An index number expresses a value relative to a base period. The base period is typically set to 100. An index of 125 means a 25% increase from the base. Index numbers are used for price indices (CPI), stock indices, and economic indicators.

Worked Example

A commodity priced at $125 today vs. $100 in the base year.

current = 125base = 100base_index = 100
  1. 01Index = (125 / 100) * 100 = 125
  2. 02Change from base = ((125/100) - 1) * 100 = 25%
  3. 03The price is 25% above the base period.

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