Common air purifier / box-fan filter sizes
Most high-airflow purifiers and DIY box-fan builds use standard imperial furnace-filter sizes, which is exactly what keeps them cheap to run. The table below lists the sizes you’ll meet most often in the UK and the builds that use them.
| Nominal size | Actual (approx) | Typically used by |
|---|---|---|
| 16×25×1" | 15.7 × 24.7 × 0.75" | 5-Fan Luggable, many Corsi-Rosenthal boxes |
| 20×25×1" | 19.7 × 24.7 × 0.75" | 7-Fan XL Luggable, larger CR boxes |
| 16×20×1" | 15.7 × 19.7 × 0.75" | compact box builds |
| 20×20×1" | 19.7 × 19.7 × 0.75" | square box-fan builds |
| 6"/6.5" round | cylindrical | personal/desktop units (e.g. Exhalaron) |
Nominal vs actual — why filters read “16×25” but measure smaller
Furnace-filter sizes are nominal: a “16×25” filter actually measures about 15.7×24.7 inches so it slides into a 16×25 slot without jamming. This catches people out when they measure a filter with a tape, get an odd figure, and assume it’s a special size. It almost never is. Always buy by the printed nominal size, not the tape-measure figure, and don’t worry about the fraction-of-an-inch difference — it is designed in.
Why thickness matters too
Depth is part of the size. A 1″ filter is standard for box-fan and Corsi-Rosenthal builds and for many purifiers; some systems take 2″ or 4″ media instead. A thicker filter has more surface area, so it can last longer and resist airflow less — but only if the slot is built for it. Never force a 2″ filter into a 1″ slot, and don’t run a 1″ filter in a deep slot where air can slip around the edges. Match all three numbers: length, width and depth.
How to measure your filter
- Read the size printed on the cardboard edge of the old filter first — it’s almost always there.
- No print? Measure length × width × thickness in inches and round to the nearest standard size.
- Thickness matters: 1″ is standard for box-fan and CR builds. Don’t force a 2″ filter into a 1″ slot.
- Check the filter sits flush and the frame seals against the housing — gaps let air bypass the media.
Round vs rectangular
Personal and desktop units sometimes use a cylindrical filter (typically 6″ or 6.5″ round) instead of a flat panel. These are still often standardised within a product family, but they are less interchangeable than flat furnace sizes, so check the exact diameter and height. For most room-sized cleaning, a flat 16×25 or 20×25 panel gives you far more media and far more choice of supplier.
Which size should I buy?
If you run a Luggable or a 16×25 Corsi-Rosenthal box, you want 16×25×1. The 7-Fan XL and 20×25 boxes take 20×25×1. Both are standard MERV-13 sizes, so you are never tied to one supplier. For the grade to choose, see HEPA vs MERV-13; for where to buy at the best lifetime cost, our where-to-buy guide; and for exact products and annual cost, the replacement filter guide.
Why standard sizes save money
The reason we keep steering people towards 16×25 and 20×25 is simple: they are mass-produced furnace-filter sizes, so dozens of manufacturers make them and prices stay low. A proprietary shape, by contrast, has one maker and one price. Sticking to a standard size means you can shop around, stock up when there’s an offer, and never face a discontinued-part problem years down the line. It is the single biggest lever on the lifetime cost of running a purifier — see the where-to-buy guide for the cheapest routes.