Fitment & Size Guide — Check Before You Order
Ordering the wrong part wastes time and money. This auto parts fitment guide walks you through the three checks that confirm a part fits your vehicle: your VIN, the OEM part number with auto parts interchange data, and the physical measurements of the part you are replacing.
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The three-way fitment check
Every product page on this site verifies compatibility three ways: your vehicle VIN, the OEM part number, and the interchange table. Use the VIN lookup in the Help Center before you order — it is the fastest way to settle "will this part fit my car".
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How to Check if a Part Fits — 3 Steps
Confirm your vehicle
Enter the full 17-character VIN for the most precise result, or select year, make, model and engine. The auto parts fitment checker on every product page matches your vehicle before you buy. Start with the VIN lookup in the Help Center.
Cross-reference the part number
Find the OEM number on the old part, then compare it against the listing. The auto parts interchange database tells you which aftermarket numbers replace the original — that is how car parts interchange is confirmed, and it is faster than guessing by eye.
Measure the part you are replacing
Compare physical auto parts dimensions — diameter, width, thickness, bolt pattern, connector type — with the specifications in the listing. The measurement steps below show exactly what to check for wheels, brakes, batteries, belts and filters.
How to Measure Auto Parts
You do not need specialist tools — a ruler, a tape measure and a caliper cover most checks. Use this auto parts measurement guide to check the five categories that cause the most fitment questions.
How to read tire size
Wheel diameter is measured in inches across the rim, and width is measured from bead seat to bead seat. The sidewall code on your tire already tells you most of it — e.g. 225/45 R17 means 225 mm width, 45% aspect ratio, 17-inch rim.
- Diameter: inches across the rim
- Width: bead seat to bead seat
- Offset: mounting face to centerline
- Bolt pattern: lug count x circle diameter
Brake rotors & pads
Auto parts rotors are sized by diameter, thickness and venting. Measure the rotor diameter across the friction surface and the pad thickness with a caliper — do not rely on brand alone, because one vehicle can use several rotor sizes.
- Rotor diameter across friction surface
- Rotor thickness (new vs min spec)
- Pad shape and friction material
- Caliper type: floating or fixed
Battery group size
Batteries are matched by BCI group size, not just cold-cranking amps. Measure length, width and height, and check the terminal layout — a battery with the right CCA but the wrong footprint will not sit in the tray.
- BCI group number (footprint)
- Length, width, height in mm
- Terminal layout: top vs side post
- CCA vs your vehicle's requirement
Belt length & width
Serpentine belts are identified by length, width and rib count. Lay the old belt flat and measure the outside length, then count the ribs on the inside — two belts that look similar can differ by a single rib.
- Outside length in mm
- Width across the ribs
- Rib count (e.g. 6PK)
- Groove profile match
Oil & air filter size
Filters are matched by diameter, height and thread pitch. Measure the outer diameter and overall height, and check the thread size on oil filters — the same engine family often uses different filters across years.
- Outer diameter in mm
- Overall height
- Thread size (oil filters)
- Seal/gasket diameter
Thread & connector
For sensors, switches and fittings, the thread size and connector type decide the fit. Check the thread diameter and pitch with a thread gauge, and match the connector shape to the harness.
- Thread diameter and pitch
- Connector type and pin count
- Overall length of the body
- Mounting orientation
Auto Parts Specifications — What the Numbers Mean
These are the auto parts specifications that decide whether a component bolts on. Match every field against the part you are replacing, not just the ones that are easy to see.
| Specification | What it is | Why it matters |
|---|---|---|
| Bolt pattern | Number of lug holes and the diameter of the circle they form, e.g. 5x114.3 | A wrong auto parts bolt pattern means the wheel cannot be fitted safely |
| Offset | Distance from the wheel mounting face to the centerline | Wrong offset pushes the wheel into the fender or suspension |
| Diameter | Width of the friction surface or opening, usually in mm or inches | Rotors, filters and pulleys must match the mating part |
| Width | Lateral measurement of the part body or friction surface | Over-width pads or belts contact adjacent components |
| Thickness | Depth of the part, often with a minimum service spec | Worn rotors and pads below spec fail on the road |
| Thread size | Diameter and pitch of the threaded portion | Sensors and fittings with the wrong thread size leak or cross-thread |
| Wheel size | Rim diameter and width the tire is designed for | An incorrect auto parts wheel size changes gearing and speedometer reading |
When a listing is missing a specification, compare the removed part with the description photo or message us — we will confirm the dimension against the catalogue before you order.
Fitment Checks by Part Category
A quick checklist for the categories where a size mismatch is the most common reason for a wrong order.
Brakes
- Auto parts brake pad size: shape, width, thickness
- Rotor diameter and venting type
- Caliper type: floating or fixed
- Front vs rear — they differ
Batteries
- Auto parts battery size: BCI group footprint
- CCA rating for your climate
- Terminal layout and polarity
- Tray height clearance
Wheels & Tires
- Bolt pattern and hub bore
- Offset and backspacing
- Rim diameter and width
- Tire sidewall code check
Belts & Filters
- Auto parts belt size: length and rib count
- Filter diameter, height, thread
- Seal/gasket diameter
- Year-range variations
Common Auto Parts Fitment Issues & Fixes
Auto parts not fitting is almost always one of three problems — check these before you return anything.
Not enough clearance
Parts that rub the fender, suspension or heat shields usually have the wrong offset, width or diameter. Auto parts clearance is measured in the gap between the part and surrounding components — if a wheel or caliper touches, stop and re-check the offset.
Wrong size ordered
Two visually similar parts can differ by a few millimetres — or one rib. A wrong size is usually caught at the measurement step, not at installation.
Part does not bolt on
Bolt holes that do not line up mean the bolt pattern or thread size is different, or the variant is for a different engine/trim of the same model.
Size Charts & Conversions
Quick reference charts for the conversions you will need while measuring — this auto parts conversion chart and size chart cover mm-to-inch and bolt pattern reading. Bookmark this page so you never convert by guesswork.
Millimetres to Inches
- 1 mm = 0.039 in
- 10 mm = 0.39 in
- 25.4 mm = 1 in
- 50 mm = 1.97 in
- 100 mm = 3.94 in
- 114.3 mm = 4.5 in
Use mm on the ruler when you can — it removes conversion error at the measurement step.
Bolt Pattern Reader
- 4 lug measure across opposite holes
- 5 lug measure odd-hole method
- 6 lug measure across opposite holes
- 8 lug measure across opposite holes
- e.g. 5x114.3 5 holes, 114.3 mm circle
An odd number of lugs has no opposite hole — measure from the center of one hole to the far edge of the opposite one.
Still Not Sure It Fits?
Send us your VIN and the old part number
Our fitment support team checks the interchange and the measurements against the catalogue, then confirms the part before you order. Every message gets a reply within 24 hours, and we answer auto parts questions in English, Spanish, Portuguese, French and Arabic.
Contact Fitment SupportFrequently Asked Questions
What is the difference between auto parts interchange and cross reference?
They describe the same idea from two sides. An auto parts interchange chart lists aftermarket numbers that replace your OEM part number; a cross reference chart does the same the other way around. Both confirm that a replacement part is a genuine match, and every product page on this site shows the interchange data before you order.
Do I need my VIN to find parts?
A 17-character VIN gives the most precise result because it encodes your engine, transmission and factory options. If you do not have the VIN handy, year, make, model and engine usually narrow it down — but the VIN removes the guesswork.
How do I measure a part?
Use a ruler, tape measure or caliper. Wheels and tires: diameter and width. Brakes: rotor diameter and thickness, pad shape. Batteries: length, width, height and terminal layout. Belts: outside length and rib count. Filters: diameter, height and thread size. The measure section above walks through each one.
What is a bolt pattern?
A bolt pattern describes the lug holes on a wheel hub, written as the number of lugs and the diameter of the circle they form — for example 5x114.3 means five lugs on a 114.3 mm circle. A wheel with the wrong auto parts bolt pattern cannot be fitted safely.
What does clearance mean in fitment?
Clearance is the gap between a part and the components around it. If a wheel or caliper rubs the fender, suspension or heat shield, the offset, width or diameter is wrong. Auto parts clearance problems show up as rubbing, vibration or a part that simply will not sit right.
How accurate is the fitment checker?
The checker verifies compatibility three ways — your VIN, the OEM part number and the interchange table. When a vehicle variant is rare or has factory options, send us the VIN and the old part number and we confirm the match before you order.