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Prepositions after decrease

Decrease in or decrease by?

Use decrease in for the thing that becomes smaller and decrease by for the amount of reduction.

Quick answer

Difference between decrease in and decrease by

Use decrease in before the category or thing affected: a decrease in traffic. Use decrease by before the amount of change: traffic decreased by 30%.

There was a decrease in unemployment last year.
Unemployment decreased by 2% in June.
Energy use decreased by 15 units overnight.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
decrease in + noun

What becomes smaller

Use in before the thing or area that experiences a reduction.

The report showed a decrease in crime.
We noticed a decrease in demand after the price rise.
2
decrease by + amount

How much something changes

Use by before percentages, numbers, or measured amounts.

Costs decreased by 8% this year.
The speed decreased by 10 kilometres per hour.
3
decrease from + starting level

Original amount before reduction

Use from to show the earlier level.

The population decreased from 2 million in 2000.
Sales decreased from last year's figures.
4
decrease to + final level

Final amount after reduction

Use to for the lower level reached after the decrease.

Output decreased to 500 units in May.
The rate decreased to 4% by autumn.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
decrease in
There was a decrease in traffic.
In introduces the thing that became smaller.
decrease by
Traffic decreased by 25%.
By introduces the amount of change.
decrease from ... to
Traffic decreased from 1,000 cars to 700.
Shows both the starting and ending levels.
decrease to
The price decreased to $40.
To introduces the final lower amount.

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using in with percentages

Use by before percentages and numerical changes.

✕ Sales decreased in 12%. ✓ Sales decreased by 12%.

Using by before the affected area

Use in before the thing that became smaller.

✕ There was a decrease by tourism. ✓ There was a decrease in tourism.

Leaving out to after a final amount

Use to before the ending level.

✕ The rate decreased 3%. ✓ The rate decreased to 3%.

Confusing by and from

By shows the size of the change; from shows the starting point.

✕ Profits decreased from 15%. ✓ Profits decreased by 15%.

Quick practice

Choose the expression that matches the intended meaning.

1. There was a decrease ___ fuel consumption.
2. Fuel consumption decreased ___ 10%.
3. The number decreased ___ 90 ___ 60.

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Frequently asked questions

What is the difference between decrease in and decrease by?

Decrease in names the thing affected, while decrease by gives the amount of change.

Can decrease be followed by to?

Yes. Use decrease to before the final lower amount, such as prices decreased to $50.

Is decrease in used with percentages?

Usually no. Percentages normally follow decrease by, not decrease in.

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