Mouse trap calculator: how many traps do you need?

How the calculator works

House mice stay close to home. The UC Statewide IPM Program says they seldom travel more than 30 feet from their nest and food, so it recommends spacing traps no more than about 10 feet apart where mice are active. The Utah State University IPM manual for rats and mice gives one trap every 6 to 10 feet along a wall, with more where there are many signs, and calls too few traps one of the most common mistakes.

The calculator turns that into a count:

• Trap locations = feet of wall with signs ÷ spacing, rounded up. Spacing is 10 feet for light activity and 6 feet for heavy activity.
• Every room or area with signs gets at least one location, even if its wall run is short.
• Double sets put two traps side by side at each location. UC IPM says pairs set 1 to 2 inches apart catch more mice that jump over obstacles, so double sets double the count.

Worked example

You find droppings along 40 feet of wall in a kitchen and a pantry, and there are a lot of them. At 6-foot spacing, 40 ÷ 6 = 6.7, which rounds up to 7 locations. Seven locations with double sets is 14 traps.

Where to put the traps

• Set traps flat against the wall, at right angles to it, with the trigger end touching the wall. Mice run along edges and cross the trigger that way.
• Put them behind objects, in dark corners and where droppings are, not in the middle of the floor.
• Think in three dimensions. The USU manual says signs on one floor mean you should also trap the rooms beside, below and above.
• Keep traps where children and pets cannot reach them, or put snap traps inside a tamper-resistant station.

If you hear scratching but cannot find droppings, start with our guide to hearing mice in the walls with no droppings. If the noise is inside a wall cavity, our guide to getting a mouse out of the wall without opening it covers trap placement near entry points. Mice also travel up rough surfaces, so read what mice can and can't climb before you decide traps only belong on the floor.

Limits

This is a starting count, not a guarantee. A large or long-running infestation, mice in a multi-unit building, or signs of rats need a licensed pest professional.