How the air filter size chart works
The rule is simple and it never changes: the actual width is the nominal width minus half an inch, the actual height is the nominal height minus half an inch, and the actual depth is 0.75 in for a 1-in nominal filter. The centimetres are the inches times 2.54, to 1 decimal: 19.5 × 2.54 = 49.5 cm, and 24.5 × 2.54 = 62.2 cm. The half inch is the frame: the filter media sits inside a cardboard frame, and the frame is what you measure when you measure the filter itself.
At the default — a 20 by 25 by 1 in nominal furnace filter — the actual width is 20 − 0.5 = 19.5 in, the actual height is 25 − 0.5 = 24.5 in, and the actual depth is 0.75 in, so the filter is 19.5 by 24.5 by 0.75 in, which is 49.5 by 62.2 cm. The reverse works the other way: an actual filter measuring 19.5 by 24.5 in came from a nominal size of 19.5 + 0.5 = 20 by 24.5 + 0.5 = 25, so buy a 20 by 25 by 1 in filter.
| Nominal (W × H × 1 in) | Actual (W × H × 0.75 in) | Actual (cm) |
|---|
| 10 × 20 | 9.5 × 19.5 in | 24.1 × 49.5 cm |
| 12 × 12 | 11.5 × 11.5 in | 29.2 × 29.2 cm |
| 14 × 20 | 13.5 × 19.5 in | 34.3 × 49.5 cm |
| 16 × 20 | 15.5 × 19.5 in | 39.4 × 49.5 cm |
| 16 × 25 | 15.5 × 24.5 in | 39.4 × 62.2 cm |
| 20 × 20 | 19.5 × 19.5 in | 49.5 × 49.5 cm |
| 20 × 25 | 19.5 × 24.5 in | 49.5 × 62.2 cm |
The worked example, line by line
Setup: nominal 20 by 25 by 1 in; reverse actual 19.5 by 24.5 in.
Line 1 — the actual width. 20 − 0.5 = 19.5 in.
Line 2 — the actual height. 25 − 0.5 = 24.5 in.
Line 3 — the actual depth. A 1-in nominal filter is 0.75 in deep, because the frame takes a quarter of an inch.
Line 4 — the centimetres. 19.5 × 2.54 = 49.5 cm and 24.5 × 2.54 = 62.2 cm.
Line 5 — the reverse lookup. An actual filter of 19.5 by 24.5 in came from a nominal size of 19.5 + 0.5 = 20 by 24.5 + 0.5 = 25, so buy a 20 by 25 by 1 in filter. An actual filter of 15.5 by 19.5 in came from a 16 by 20 by 1 in filter.
Every branch, checked
16 by 20: the actual size is 16 − 0.5 = 15.5 in by 20 − 0.5 = 19.5 in, which is 39.4 by 49.5 cm.
14 by 20: the actual size is 14 − 0.5 = 13.5 in by 20 − 0.5 = 19.5 in, which is 34.3 by 49.5 cm.
12 by 12: the actual size is 12 − 0.5 = 11.5 in by 12 − 0.5 = 11.5 in, which is 29.2 by 29.2 cm — a square filter for a small furnace.
16 by 25: the actual size is 16 − 0.5 = 15.5 in by 25 − 0.5 = 24.5 in, which is 39.4 by 62.2 cm.
20 by 20: the actual size is 20 − 0.5 = 19.5 in by 20 − 0.5 = 19.5 in, which is 49.5 by 49.5 cm — a square filter for a large furnace.
10 by 20: the actual size is 10 − 0.5 = 9.5 in by 20 − 0.5 = 19.5 in, which is 24.1 by 49.5 cm — the narrowest standard size, for a small return vent.
18 by 24, not a standard size: the computed actual size is 18 − 0.5 = 17.5 in by 24 − 0.5 = 23.5 in, which is 44.5 by 59.7 cm — but 18 by 24 is not a standard size, so do not round it to the nearest standard size: measure your existing filter and buy the exact size, because a filter must fit exactly, and a filter half an inch too big will not go in at all.
Reverse — the actual sizes: an actual filter of 15.5 by 19.5 in came from a nominal 16 by 20 by 1 in filter, and an actual filter of 19.5 by 24.5 in came from a nominal 20 by 25 by 1 in filter. Add half an inch to each side you measured, and that is the size to buy.
What the defaults assume
The chart covers the seven standard 1-in nominal sizes: 10 by 20, 12 by 12, 14 by 20, 16 by 20, 16 by 25, 20 by 20 and 20 by 25. It assumes the nominal depth is 1 in, so the actual depth is 0.75 in; 2-in and 4-in/5-in media filters share the same face sizes, so a 20 by 25 by 4 in filter is still 19.5 by 24.5 in on the face, only deeper. It assumes the half-inch rule, which is the industry standard for pleated filters: a filter that measures exactly its nominal size is either a custom size or a different kind of filter. This is explicitly NOT a MERV-rating or furnace-compatibility chart: MERV is how fine the filter catches, and compatibility is whether your furnace takes that size at all — this chart only turns one size into the other. Rounding convention: centimetres are the inches times 2.54, to 1 decimal, and all figures are computed from the inch sizes, rounded only for display.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Actual size | nominal − 0.5 in each side | 19.5 × 24.5 in, depth 0.75 in. |
| Actual size (cm) | inches × 2.54 | 49.5 × 62.2 cm. |
| 16 × 20 nominal | same rule | 15.5 × 19.5 × 0.75 in actual (39.4 × 49.5 cm). |
| 14 × 20 nominal | same rule | 13.5 × 19.5 × 0.75 in actual (34.3 × 49.5 cm). |
| 12 × 12 nominal | same rule | 11.5 × 11.5 × 0.75 in actual (29.2 × 29.2 cm). |
| 16 × 25 nominal | same rule | 15.5 × 24.5 × 0.75 in actual (39.4 × 62.2 cm). |
| 20 × 20 nominal | same rule | 19.5 × 19.5 × 0.75 in actual (49.5 × 49.5 cm). |
| 10 × 20 nominal | same rule | 9.5 × 19.5 × 0.75 in actual (24.1 × 49.5 cm). |
| 18 × 24 nominal | same rule, not standard | ; measure your existing filter. |
| Reverse: 19.5 × 24.5 actual | actual + 0.5 in each side | Actual 19.5 × 24.5 in → nominal 20 × 25 × 1 in. |
| Reverse: 15.5 × 19.5 actual | actual + 0.5 in each side | Actual 15.5 × 19.5 in → nominal 16 × 20 × 1 in. |
Who uses this
- The homeowner changing the furnace filter: they learn their 20 by 25 by 1 in filter is really 19.5 by 24.5 by 0.75 in — so they stop measuring the slot and start reading the size printed on the old filter, which is the size to buy.
- The renter with no old filter to copy: they learn an actual slot of 15.5 by 19.5 in takes a 16 by 20 by 1 in filter — so they add half an inch to each side they measured and buy the right size first time.
- The DIYer with an odd size: they learn 18 by 24 is not a standard size — so they measure the old filter and order the exact size instead of forcing a standard filter that will not fit.
- The landlord stocking filters for several rentals: they learn the seven standard sizes cover most furnaces, from 10 by 20 to 20 by 25 — so they keep one of each on the shelf and never guess at the store.
Accuracy & limitations
The chart is exact for the standard sizes: the actual size is always the nominal size minus half an inch on each side, and the centimetres are the inches times 2.54, to 1 decimal. What the chart cannot do is measure your filter slot for you: measure the filter you already have, not the slot, because the slot is bigger than the filter on purpose. It assumes a 1-in nominal filter, so the actual depth is 0.75 in; 2-in and 4-in/5-in media filters share the same face sizes, so only the depth changes. It is not a MERV-rating or furnace-compatibility chart: MERV is how fine the filter catches, and compatibility is whether your furnace takes that size at all. For the furnace itself, see furnace filter size chart; for the heating system, see heat pump vs furnace.
Cite this reference tableBright Side Kit. “Air Filter Size Chart: What Size Filter Do I Need?” brightoolbox.com, 2026. Rule: actual width = nominal width − 0.5 in; actual height = nominal height − 0.5 in; actual depth 0.75 in for a 1-in nominal filter; cm = in × 2.54, 1 decimal; reverse: nominal = actual + 0.5 in on each side. Rows: 10×20 → 9.5×19.5 in (24.1×49.5 cm); 12×12 → 11.5×11.5 in (29.2×29.2 cm); 14×20 → 13.5×19.5 in (34.3×49.5 cm); 16×20 → 15.5×19.5 in (39.4×49.5 cm); 16×25 → 15.5×24.5 in (39.4×62.2 cm); 20×20 → 19.5×19.5 in (49.5×49.5 cm); 20×25 → 19.5×24.5 in (49.5×62.2 cm). Lookup: 20 × 25 × 1 in nominal → 19.5 × 24.5 × 0.75 in actual (49.5 × 62.2 cm). Non-standard: 18×24 → 17.5×23.5 in (44.5×59.7 cm), not a standard size — measure your existing filter, do not snap to a standard size. Reverse: actual 15.5×19.5 in → nominal 16×20×1; actual 19.5×24.5 in → nominal 20×25×1. 2-in and 4-in/5-in media filters share the same face sizes. This is not a MERV-rating or furnace-compatibility chart.Bright Side Kit. "Air Filter Size Chart: What Size Filter Do I Need?" https://brightoolbox.com/air-filter-size-chart/ Accessed October 24, 2026.
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