An air purifier is a fan and a gasket. If the filter does not seal against the housing, air takes the easier path around it and the unit cleans almost nothing — while making exactly the same noise and using exactly the same electricity. That is why “it fits” is not the same as “it works”.
This guide is the four-check routine that guarantees the right cartridge, plus the model-number lookups for the most common apartment purifiers.
- Model number from the base of the unit — not from the brand on the front.
- Filter dimensions — diameter and height, measured with a ruler.
- Media spec — must say True HEPA or H13, plus a carbon layer if you want odour control.
- Gasket fit — the cartridge should need a light press to seat, with no visible gap.
On this page
1. Find your real model number
The brand name on the front of the unit is marketing. The model number on the base or the back label is what determines the cartridge. They are often different things.
- Turn the unit upside down. The label is almost always on the bottom.
- Check inside the filter door. Some brands print the cartridge part number on the housing itself.
- Photograph the label. You will need it when ordering, and labels fade.
- Search model + “replacement filter”, not brand + “filter”. The model narrows it instantly.
A common trap: a “Levoit Core 300” and a “Levoit Core 300S” use the same cartridge, but the 400S does not. One letter, different part.
2. Measure before you buy
Cartridges are almost-cylinders. Measure the outside diameter at the widest point and the total height including any base ring. A difference of 3–4 mm is enough to break the seal.
| Measurement | Where to take it | Tolerance that matters |
|---|---|---|
| Outside diameter | Across the widest point of the media | ±2 mm |
| Height | Base ring to top edge | ±3 mm |
| Base ring diameter | The step that seats in the housing | ±1 mm |
| Inner bore | If the cartridge slides over a post | ±2 mm |
Keep your old cartridge. When the new one arrives, stand them side by side on a table. If the new one is visibly shorter or narrower, send it back before you fit it — you have caught a mismatch that would otherwise show up as “this purifier is rubbish” six weeks later.
3. Read the spec, not the marketing
Filter listings are full of words that mean nothing. Here is the translation table.
| Marketing wording | What it actually means | Buy it? |
|---|---|---|
| True HEPA / H13 | 99.97% of particles at 0.3 microns | Yes |
| HEPA-type / HEPA-like | Uncertified media, unknown efficiency | Only as a cheap pre-filter |
| 99% filtration | 99% of what, at what size? Usually large dust | No, unless nothing else fits |
| Activated carbon | Adsorbs odours and VOCs | Yes, for cooking smells |
| Carbon pre-filter pad | Thin carbon sheet, saturates quickly | Weak — better than nothing |
| Washable carbon honeycomb | Reusable carbon layer with real mass | Yes — best for odour control |
4. Check the seal — this is the step everyone skips
Fit the cartridge and look around its perimeter with the unit open and running. There should be no visible gap and no obvious bypass path. Then do the tissue test: hold a tissue near the edges of the housing while the fan runs on high. It should be pulled toward the intake, not flutter freely at the sides.
- A light press to seat is right. If it drops in without contact, the seal is decorative.
- No rattle when the unit is running. Movement means a gap that grows with vibration.
- No musty smell after an hour. If a brand-new filter smells, it has been stored damp.
Common model lookups for apartment purifiers
| Purifier | Cartridge to look for | Notes |
|---|---|---|
| Levoit Core 300 / 300S | Core 300 replacement filter | Same cartridge for both. Genuine 3-in-1 HEPA + carbon. |
| Levoit Core 400S | Core 400S replacement filter | Not interchangeable with the 300 range. |
| Coway AP-1512HH | AP-1512HH filter set | Buy the set — HEPA and carbon are separate layers. |
| Winix 5500-2 | Filter 115115 | Includes a washable carbon layer; HEPA is a separate annual item. |
| Blueair Blue Pure 211+ | Blue Pure 211+ filter | Take the genuine one; the housing seal is fussy. |
| GermGuardian AC4825 | FLT4825 | Common and cheap, but the carbon layer is thin. |
Levoit Core 300 replacement filter — start here
The most common apartment purifier in the US, so the cartridge is cheap and always in stock. Genuine ones seal properly.
Check Price on Amazon →
Winix 115115 filter set — for odour-heavy kitchens
If you fry often, the washable carbon layer in the 115115 set handles cooking odours far better than thin carbon pads.
Check Price on Amazon →When a “compatible” filter is a bad idea
- The price is less than half genuine and the listing will not state a HEPA grade.
- The listing shows one photo reused across many models. That is a sign of a generic size, not a fit.
- Reviews mention rattle or a gap. Bypass is the number one complaint on cheap cartridges.
- You have allergies or asthma. With a health reason, pay for certified media — the difference is measurable.
Find the model number on the base, measure the old cartridge, insist on True HEPA or H13, and verify the seal. Those four steps eliminate essentially every wrong-filter purchase.
Frequently asked questions
How to check a filter really fits
| Check | How to confirm | Why it matters |
|---|---|---|
| Dimensions | Measure length, width and thickness in mm | A few millimetres changes the seal |
| Frame profile | Compare the outer lip shape | Poor sealing lets air bypass the filter |
| Filter count | Count the layers in the original | Multi-layer units need matching sets |
| Model list | Confirm your model is named, not implied | 'Fits most' is not a fit |
| Certification | Look for a HEPA efficiency figure | HEPA-type is not HEPA |
Air purifiers are designed around a specific filter geometry. The fan is sized to push a particular volume of air through a particular area of filter media at a particular pressure drop. Change the media and you change that balance: a denser filter starves the fan and cuts the clean air delivery rate, while a thinner one lets particles through. This is why a filter that physically fits can still leave the room dirtier than before.
Housing dimensions include clearance and the sealing lip, so they are always larger than the filter itself. Measure the filter you are removing, and if a listing quotes only 'fits model X', cross-check the dimensions against your old part before ordering.
