To determine the size of a furnace filter, measure the current filter and the filter slot. You may find a furnace filter's measurements printed along one edge of the current filter.
- Locate the current furnace filter
To find a furnace filter, open the furnace cover. The filter in most furnaces is usually positioned in the blower compartment, which is in the bottom part of a furnace.
- Remove the furnace filter
Take out the furnace filter by sliding it out of its slot. For vertical furnace filters, slide the filter up or down and then out. For horizontal furnace filters, slide the filter out through one of the open sides.
- Measure the furnace filter
With measuring tape, measure the width, height and thickness of the furnace filter.
- Note the furnace filter's measurements
Write the furnace filter's measurements in inches on a sheet of paper.
- Measure the furnace filter slot
With a measuring tape, measure the width, height and depth of the furnaces filter slot.
- Write down the measurements
Write the measurements of the filter slot in inches on a piece of paper.
- Use measurements when selecting a new furnace filter
Take both sets of measurements with you when shopping for a new furnace filter. If you cannot find a furnace filter that exactly matches the measurements you wrote down, select one that is closest in size, but not larger than your measurements.