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Overlap Percentage Calculator

Written by the percentages.co.uk team. Reviewed for accuracy.

This calculator finds the percentage overlap between two numerical ranges or two sets of items. For ranges, it calculates the overlap as a fraction of the combined span. For item sets, it uses the Jaccard similarity index: items in common divided by items in total.

Takes about 30 secondsUpdated 17 May 2026

How it works

Overlap percentage measures how much two ranges or sets share in common, expressed as a fraction of their combined total coverage. It is used in scheduling (do two events overlap?), data analysis (how similar are two customer segments?) and geography (what area do two regions share?).

The formula

Range overlap % = (Overlap / Union) × 100

Set overlap % = (|Intersection| / |Union|) × 100 (Jaccard index)

Why this works: Dividing the shared portion by the combined total (union) gives a measure that is 0 when there is no overlap at all, and 100% when the two ranges or sets are identical. The Jaccard index is widely used in data science, ecology and text comparison for exactly this reason.

Worked examples

Two salary bands: Band A is £25,000-£40,000 and Band B is £35,000-£55,000. What is the overlap?

  1. Overlap: £35,000 to £40,000 = £5,000
  2. Union: £25,000 to £55,000 = £30,000
  3. Overlap %: (5,000 / 30,000) × 100 = 16.67%

Answer: 16.67% overlap

Two shifts overlap: Shift A is 08:00-16:00 (480 min) and Shift B is 14:00-22:00 (480 min). What percentage do they share?

  1. Overlap: 14:00-16:00 = 120 minutes
  2. Union: 08:00-22:00 = 840 minutes
  3. Overlap %: (120 / 840) × 100 = 14.29%

Answer: 14.29% overlap

Two customer segments: Segment A has features [A, B, C, D], Segment B has [C, D, E, F]. What is the Jaccard similarity?

  1. Intersection: {C, D} = 2 items
  2. Union: {A, B, C, D, E, F} = 6 items
  3. Jaccard: (2 / 6) × 100 = 33.33%

Answer: 33.33% overlap (Jaccard similarity)

Two age ranges: 18-35 and 25-45. What is the overlap percentage?

  1. Overlap: 25-35 = 10 years
  2. Union: 18-45 = 27 years
  3. Overlap %: (10 / 27) × 100 = 37.04%

Answer: 37.04% overlap

Two product tag sets: Product A: [organic, vegan, gluten-free, UK-made]. Product B: [organic, UK-made, fair-trade, sustainable]. What is their similarity?

  1. Intersection: organic, UK-made = 2 items
  2. Union: 6 unique items
  3. Jaccard: (2 / 6) × 100 = 33.33%

Answer: 33.33% similarity

When to use this

  • Scheduling and HR: Checking how much two work schedules or shift patterns overlap to identify handover windows or potential conflicts.
  • Market research: Measuring how similar two customer segments or survey respondent groups are based on their shared characteristics.
  • SEO and content marketing: Comparing keyword lists for two pages to identify cannibalisation risk (high overlap) or gaps in coverage (low overlap).
  • Property and planning: Checking whether two postal code ranges, price bands or catchment areas share territory.

Understanding the result

A 0% overlap means the two ranges or sets share nothing in common. A 100% overlap means they are identical. For set comparison, the Jaccard index tends to be lower than intuition suggests because the denominator is the total number of unique items across both sets, not just one set.

For ranges, a small absolute overlap may represent a large percentage if the overall span is narrow, and vice versa. Consider both the absolute overlap amount and the percentage.

Related concepts

➡ To understand what percentage one set or range is of a total, the frequency percentage calculator shows each category as a share of the total. ➡ For calculating how much of a time period two events share, the percentage to minutes calculator converts time fractions into minutes. ➡ To find the gap between two percentage values, use the percentage points calculator to find the difference between two rates.

How to do this in Excel

Range overlap: =(MAX(0,MIN(B1,D1)-MAX(A1,C1))/((MAX(B1,D1)-MIN(A1,C1))))*100

For two ranges with A1:B1 being the first range and C1:D1 the second. Returns 0 if there is no overlap. For set overlap, use SUMPRODUCT with array formulas or Power Query.

How to do this without a calculator

For numerical ranges: find the highest start point and lowest end point of both ranges to get the overlap. Find the lowest start and highest end to get the union. Divide overlap by union and multiply by 100. If the overlap start exceeds the overlap end, there is no overlap.

Common mistakes

Dividing overlap by one range instead of the union

Overlap percentage must use the combined union as the denominator. Dividing by just one of the ranges inflates the percentage and makes the result asymmetric.

Treating case-sensitive and case-insensitive set items differently

When comparing item sets, make sure your items are in a consistent case. "London" and "london" will be counted as different items in a case-sensitive comparison, reducing the measured overlap.

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