How to Find Your AC Filter Size

There are three easy ways to determine the correct filter size for your HVAC system:

Method 1: Check the Existing Filter

The fastest way is to look at the filter currently in your system. The size is printed on the frame — typically in a format like 16x25x1 or 20x20x4, measured in inches (length x width x depth). This is the nominal size, which is the industry-standard label. The actual dimensions may be slightly smaller (usually 1/4 to 1/2 inch less on each side).

Method 2: Measure the Filter Slot

If there is no filter installed or the label has worn off, measure the filter slot opening on your return air grille or air handler. Measure the length, width, and depth in inches. Round up to the nearest standard size.

Method 3: Check Your HVAC Manual

Your furnace or air handler manual specifies the correct filter size. If you do not have the manual, search for your unit model number online — most manufacturers publish specifications digitally.

Most Common AC Filter Sizes

These are the most frequently purchased residential air filter sizes in the United States. If your home uses one of these sizes, you are in luck — they are widely available and competitively priced:

Standard 1-Inch Filters

• 16x20x1 — Very common in smaller systems and apartments
• 16x25x1 — One of the top 3 most popular sizes nationwide
• 20x20x1 — Common in mid-size residential systems
• 20x25x1 — The single most popular filter size in American homes
• 20x30x1 — Found in larger residential systems
• 24x24x1 — Common in commercial and some residential units
• 14x20x1 — Older systems and some mobile homes
• 14x25x1 — Less common but still standard

Deep-Pleat Filters (4-5 Inches)

• 16x25x4 — Carrier, Bryant, and other brands with media cabinets
• 20x25x4 — Honeywell, Lennox, and other 4-inch filter systems
• 20x25x5 — Honeywell, Aprilaire, and similar whole-house filter cabinets

We stock the most popular sizes. Check our air filter collection for current pricing and multi-pack deals.

Understanding MERV Ratings

MERV stands for Minimum Efficiency Reporting Value. It measures how effectively a filter captures airborne particles. The scale runs from 1 to 20, but residential systems typically use filters rated MERV 8 through MERV 13.

MERV Rating Breakdown

• MERV 1-4: Basic fiberglass filters. Catch large particles like dust bunnies and carpet fibers. Minimal air quality improvement. Better than nothing.
• MERV 5-8: Pleated filters. Capture dust, pollen, mold spores, and pet dander. This is the sweet spot for most homes — good filtration without restricting airflow. Our MERV 8 filters are the most popular choice.
• MERV 9-12: Higher efficiency pleated filters. Capture finer particles including Legionella bacteria, humidifier dust, and auto emissions. Great for allergy sufferers. Our MERV 11 deep-pleat filters offer excellent filtration with a long service life.
• MERV 13-16: Hospital-grade filtration. Captures bacteria, tobacco smoke, and sneeze droplets. May restrict airflow in older systems — check with your HVAC manufacturer before using MERV 13+ in a standard system.
• MERV 17-20: HEPA territory. Used in hospitals and cleanrooms. Not designed for standard residential HVAC systems.

What MERV Rating Should You Use?

For most homes, MERV 8 is the best balance of filtration and airflow. It captures the particles that matter — dust, pollen, pet dander, mold — without making your system work harder. If someone in your household has allergies or asthma, step up to MERV 11. Only use MERV 13 or higher if your system is specifically designed for it.

How Often Should You Change Your AC Filter?

• 1-inch filters: Every 30-90 days
• 4-inch filters: Every 6-12 months
• 5-inch filters: Every 9-12 months

Change more frequently if you have:

• Pets (pet hair and dander clog filters fast)
• Allergies or asthma
• A dusty environment or construction nearby
• The AC running constantly during peak summer

A good rule of thumb: check your filter monthly. Hold it up to a light source — if you cannot see light through it, replace it. A clean filter is the single cheapest thing you can do to keep your AC running efficiently and extend the life of your system.

Filter Buying Tips

• Buy in bulk. Multi-packs save money and ensure you always have a replacement ready. Our 6-packs are priced to cover half a year or more.
• Set a reminder. Put a recurring reminder on your phone to check the filter. Out of sight, out of mind is how filters get neglected.
• Do not wash disposable filters. Pleated filters are designed for single use. Washing them damages the media and reduces effectiveness.
• Arrow points toward the blower. Every filter has an airflow direction arrow printed on the frame. The arrow should point toward the furnace or air handler — in the direction of airflow.

Ready to stock up? Browse our complete filter selection or contact us if you need help finding the right size for your system.

1. Dirty Air Filter (Most Common Cause)

A clogged air filter is the number one reason AC systems underperform. When the filter is packed with dust and debris, it restricts airflow across the evaporator coil. The coil gets too cold, ice forms on it, and your AC blows warm air even though the compressor is running.

The fix: Check your air filter. If you cannot see light through it, replace it immediately. Standard 1-inch filters should be changed every 60-90 days. If you have pets, change them monthly. Browse our air filter collection for every common size.

2. Bad Capacitor

The capacitor gives your compressor and fan motor the electrical jolt they need to start. When it fails, you will hear the outdoor unit humming or clicking but the compressor will not engage. A bad capacitor is the most common part failure in residential AC systems, especially in hot climates where they work overtime.

The fix: Replace the capacitor. This is a $12-17 part that takes about 15 minutes to swap. Read our complete AC capacitor replacement guide for step-by-step instructions. Shop capacitors here.

3. Tripped Breaker or Blown Fuse

Sometimes the solution is embarrassingly simple: the breaker tripped. Your AC runs on its own dedicated circuit, and a power surge, lightning strike, or momentary overload can trip it. Also check the disconnect box next to your outdoor unit — some use fuses that can blow.

The fix: Check your electrical panel for a tripped breaker and reset it. If the fuse in your disconnect box is blown, replace it. If the breaker trips again immediately, stop — you have an electrical problem that needs a professional. We carry replacement fuses and disconnect boxes.

4. Frozen Evaporator Coil

If you see ice forming on the refrigerant lines or the indoor coil, your evaporator coil is freezing up. This is usually caused by restricted airflow (dirty filter, closed vents) or low refrigerant. A frozen coil cannot absorb heat, so your AC blows warm or lukewarm air.

The fix: Turn off the AC and let the coil thaw completely — this can take several hours. Replace the air filter, open all supply vents, and make sure nothing is blocking return air grilles. If the coil freezes again after restarting, you likely have a refrigerant leak that needs professional repair.

5. Bad Contactor

The contactor is an electrical switch that turns the compressor and fan motor on and off. Over time, the contacts pit, burn, and corrode — especially in dusty or humid environments. A worn contactor may cause intermittent cooling, loud buzzing from the outdoor unit, or a complete failure to start.

The fix: Replace the contactor. A 2-pole 30 amp contactor fits most residential systems and costs under $17. This is a straightforward swap that takes about 20 minutes. Shut off all power before starting.

6. Dirty Condenser Coil

Your outdoor unit releases heat through the condenser coil. When that coil is coated in dirt, grass clippings, cottonwood seeds, or pet hair, it cannot dissipate heat efficiently. The system works harder, runs longer, and may not cool your home to the set temperature.

The fix: Turn off the unit and spray the condenser coil with a garden hose from the inside out. Use a gentle stream — not a pressure washer. For heavy buildup, use a commercial coil cleaner spray first. Do this at least once a year, ideally at the start of cooling season.

7. Low Refrigerant (Leak)

If your AC is low on refrigerant, it does not have enough cooling capacity to keep up with demand. Signs include warm air from the vents, ice on the refrigerant lines, a hissing or bubbling sound near the outdoor unit, and higher-than-normal electric bills. AC systems are sealed — if the refrigerant is low, there is a leak somewhere.

The fix: This one requires a licensed HVAC technician. They will find and repair the leak, then recharge the system with the correct amount of refrigerant. For techs, we carry R-410A refrigerant, manifold gauge sets, and leak detectors.

Quick Troubleshooting Checklist

Before calling a technician, run through this checklist:

• Is the thermostat set to COOL and set below room temperature?
• Is the air filter clean?
• Is the breaker on? Is the disconnect box engaged?
• Is the outdoor unit running? Can you hear the compressor and fan?
• Are all supply vents open and unblocked?
• Is the condenser coil clean and free of debris?
• Are there any ice formations on the refrigerant lines?

If you have checked everything on this list and your AC still is not cooling, it is time to call a pro. But odds are good that one of the seven fixes above will get your system back up and running — often for less than $30 in parts.

Need parts? Browse our full AC repair parts catalog or contact us for help finding the right component for your system.

What Does an AC Capacitor Do?

Your AC capacitor stores electrical energy and delivers a boost of power to start the compressor and fan motor. Think of it like a battery that charges and discharges rapidly. Most residential AC units use a dual run capacitor — a single canister that powers both the compressor and the condenser fan motor simultaneously.

Capacitors are rated in microfarads (MFD or µF) and volts. A typical residential AC capacitor might be rated 35/5 MFD 370/440V, where 35 is the compressor side and 5 is the fan motor side.

Signs Your AC Capacitor Is Bad

• AC unit hums but does not start — the compressor tries to kick on but cannot get the electrical boost it needs
• Fan spins slowly or not at all — the fan motor side of the capacitor may have failed
• AC shuts off randomly — a weak capacitor may work intermittently before failing completely
• Capacitor looks swollen or leaking — a bulging top or oily residue is a dead giveaway
• Clicking sounds from the outdoor unit — the contactor engages but the compressor cannot start
• Higher than normal electric bills — a weak capacitor makes the compressor work harder

Safety Warning — Read This First

Capacitors store a dangerous electrical charge even when the power is off. Before touching anything inside your AC unit:

1. Turn off the AC at the thermostat
2. Turn off the breaker for the outdoor unit at your electrical panel
3. Turn off the disconnect box next to the outdoor unit
4. Wait at least 5 minutes for residual charge to dissipate
5. Use an insulated screwdriver to short across the capacitor terminals (touch the screwdriver blade across C to FAN and C to HERM terminals) to safely discharge any remaining energy

If you are not comfortable working with electrical components, call a licensed HVAC technician. A capacitor replacement typically costs $150-300 from a pro, which is money well spent if you are unsure about the process.

How to Test an AC Capacitor with a Multimeter

You will need a multimeter with a capacitance (MFD/µF) setting. Most digital multimeters under $30 include this feature.

1. Shut off all power and discharge the capacitor as described above
2. Remove the access panel on the side of your outdoor condensing unit (usually held by one or two screws)
3. Locate the capacitor — it is a silver or black cylindrical canister, usually mounted with a bracket near the top of the unit
4. Note the ratings printed on the side (for example, 45/5 MFD 370/440V)
5. Disconnect the wires — take a photo first so you remember which wire goes where. Terminals are labeled HERM (compressor), FAN, and C (common)
6. Set your multimeter to capacitance mode (the symbol looks like -|(- )
7. Test the compressor side: Touch probes to HERM and C terminals. The reading should be within 5-10% of the rated value. A 45 MFD capacitor should read between 40.5 and 49.5 MFD.
8. Test the fan side: Touch probes to FAN and C terminals. A 5 MFD side should read between 4.5 and 5.5 MFD.

If either reading is more than 10% off from the rated value, the capacitor needs to be replaced. If the multimeter reads zero or OL (open line), the capacitor is completely dead.

How to Replace Your AC Capacitor

1. Buy the correct replacement — match the MFD rating and voltage rating exactly. The voltage can be equal to or higher than the original (a 440V capacitor can replace a 370V), but the MFD values must match. Check our capacitor collection for the most common sizes.
2. Disconnect the old capacitor — pull the wire connectors straight off the terminals. Do not yank the wires.
3. Remove the mounting bracket — usually a single screw or metal strap
4. Install the new capacitor — secure it in the mounting bracket
5. Reconnect the wires using your photo as reference: HERM goes to the compressor wire, FAN goes to the fan motor wire, C (common) goes to the common/power wire
6. Replace the access panel
7. Restore power — turn on the disconnect, the breaker, and the thermostat
8. Test the system — the AC should start up within a minute. Listen for smooth operation.

When to Call a Professional

Replace the capacitor yourself if the job is straightforward — matching ratings, swapping the part, done. But call a licensed HVAC technician if:

• The new capacitor fails quickly (could indicate a deeper electrical problem)
• You see burned wires or melted connectors
• The compressor still will not start after replacing the capacitor (could be a bad compressor or contactor)
• You are not confident working around high-voltage electrical components

A capacitor costs under $20. A new compressor costs $1,500+. When in doubt, have a pro diagnose the issue to make sure you are fixing the right problem.

Common AC Capacitor Sizes

Here are the most common dual run capacitor sizes for residential AC units:

• 35/5 MFD 370/440V — Common in 1.5-2 ton units
• 40/5 MFD 370/440V — Common in 2-2.5 ton units
• 45/5 MFD 370/440V — The most common size overall (2.5-3.5 ton)
• 50/5 MFD 370/440V — Common in 3.5-4 ton units
• 55/5 MFD 370/440V — Common in 4-5 ton units
• 60/5 MFD 370/440V — Common in 5 ton units and heat pumps

Not sure what size you need? Check the label on your existing capacitor or look up your AC unit model number. You can also contact us and we will help you find the right part.