What thread size means and why you need it
Thread size is the measurement of the spiral grooves on a bolt, screw, or fitting that allow it to twist into a hole or onto another piece. When you need to replace a fastener or find a matching part, the thread size tells you what will actually fit. It is not the same as the diameter of the bolt head — it is the diameter of the threaded shaft itself, plus how tightly the spirals are spaced.
You need thread size when you are replacing a broken bolt, finding the right nut, buying a new faucet aerator, or matching a fitting to existing plumbing or equipment. Guessing wrong means the part either will not turn at all or will spin loose without holding. Thread size is usually printed on the fastener itself, but if the marking has worn away or you are looking at an old part, you will need to measure it.
Key Takeaways
- Thread size has two numbers: the diameter (how wide the shaft is) and the pitch (how far apart the spirals are), written as 1/4"-20 or M6x1.0 depending on the system used.
- Most fasteners in the United States use imperial measurements (fractions of an inch), while metric fasteners use millimeters and are common in cars, appliances, and imported tools.
- The fastest way to find thread size is to read the marking stamped or printed on the fastener itself, usually on the head of a bolt or the shaft of a screw.
- If the marking is worn away, you can measure the shaft diameter with calipers and count the threads per inch, or take the fastener to a hardware store to match it against a thread gauge.
- Imperial and metric threads are not interchangeable — a metric bolt will not fit into an imperial hole, even if the diameters look similar.
Reading the marking on the fastener
Most bolts, screws, and fasteners have a marking stamped or printed on them that tells you the thread size directly. On a bolt, look at the head — you will see a number or series of numbers and letters. On a screw, the marking is usually on the shaft just below the head. The marking follows one of two systems: imperial (used in the United States and some other countries) or metric (used almost everywhere else).
An imperial marking looks like 1/4"-20 or 10-24. The first number is the diameter of the shaft in fractions of an inch or in a gauge number. The second number is the pitch — how many threads fit in one inch. A metric marking looks like M6x1.0 or M8x1.25. The M means metric, the first number is the shaft diameter in millimeters, and the number after the x is the pitch in millimeters.
If you see a marking but it is unclear, write down exactly what you see and search for it online along with the word "fastener" or "bolt". If there is no marking at all, move to the measurement method below.
Measuring the shaft diameter with calipers
If the marking is worn away or missing, you can measure the thread size yourself using a caliper — a tool that measures small distances. A digital caliper is easiest to read and costs between five and fifteen dollars at any hardware store. Measure the diameter of the threaded shaft itself, not the bolt head. Place the caliper jaws around the shaft where the threads are, close it gently until it touches, and read the number on the display.
Write down the measurement in both inches and millimeters if your caliper shows both. If the measurement is 0.250 inches, that is 1/4 inch. If it is 6 millimeters, that is metric. This tells you which system the fastener uses. However, the diameter alone is not enough — you also need the pitch (the spacing of the threads) to order a replacement.
Counting threads per inch or per 25 millimeters
Once you know the diameter, you need to find the pitch. For imperial fasteners, count how many thread peaks fit in one inch of the shaft. Place a ruler against the threaded part and count the ridges in a one-inch span. For metric fasteners, count the threads in 25 millimeters — this is easier than counting in one millimeter. Write down the number you count.
Common imperial pitches are 20, 24, 28, and 32 threads per inch. Common metric pitches are 1.0, 1.25, 1.5, and 2.0 millimeters. Once you have the diameter and the pitch, you have the full thread size. A 1/4"-20 bolt is 1/4 inch in diameter with 20 threads per inch. An M6x1.0 bolt is 6 millimeters in diameter with 1.0 millimeter pitch.
Using a thread gauge to match fasteners
If counting threads is difficult or you want to be certain, bring the fastener to a hardware store and ask to use a thread gauge. A thread gauge is a set of metal leaves, each with a different thread pattern cut into it. You press each leaf against the threads of your fastener until one fits snugly without gaps. The leaf that fits tells you the exact thread size.
Many hardware stores will let you use their gauge for free, and some staff can read the size for you if you hand them the fastener. This method works for both imperial and metric threads and takes less than a minute. If the store does not have a gauge, ask if they sell thread gauges — they are inexpensive and useful to keep at home if you work with fasteners regularly.
Understanding imperial versus metric threads
The two systems are not interchangeable. An imperial bolt will not screw into a metric hole, and a metric bolt will not fit an imperial hole, even if the diameters are very close. A 1/4-inch bolt (0.250 inches) and an M6 bolt (6 millimeters, or 0.236 inches) look nearly identical in size, but they will not work together.
If you are replacing a fastener, match the system to what is already there. If the original fastener has a marking, use that system. If you measured the diameter and it came out in a clean fraction of an inch (like 1/4, 3/8, or 1/2), it is imperial. If it came out in whole or half millimeters (like 6, 8, or 10), it is metric. When in doubt, take the original fastener to the hardware store and ask the staff which system it is.
What to do when you have the thread size
Once you know the thread size, you can order a replacement fastener online, buy one at a hardware store, or find a matching fitting for plumbing or equipment. Write down the size clearly — for example, "1/4-20 bolt, 1 inch long" or "M6x1.0 screw" — and include it in any search or conversation with a store employee. If you are unsure whether you need a bolt, screw, stud, or other type of fastener, describe what you are fastening and ask for a recommendation.
Keep the original fastener if you can, because it is the most reliable reference. If you are ordering online and the listing does not show a picture of the fastener head, check the product description for the thread size marking to make sure it matches what you wrote down.
Frequently Asked Questions
Can I use an imperial bolt in a metric hole if the diameters are close?
No. Even if the diameters look nearly the same, the thread pitch is different, and the bolt will either not turn or will cross-thread and strip the hole. Always match the system — imperial to imperial, metric to metric.
What if I measure the diameter but the pitch does not match any common size?
Uncommon pitches exist but are rare in consumer hardware. Double-check your thread count by measuring a longer span — count threads in two inches instead of one, then divide by two. If the count is still unusual, take the fastener to a hardware store and ask a staff member to check it with a gauge.
Do I need to know the length of the fastener to find the thread size?
No. Thread size is only the diameter and pitch of the threads. Length is a separate measurement. You need both to order a replacement, but length does not affect how you find the thread size.
What does the marking on a bolt head mean if it is not a number?
Markings on bolt heads often show the material grade or strength, not the thread size. Grade markings are lines, dots, or letters stamped into the head. The thread size is usually marked on the shaft or printed elsewhere on the fastener. If you see only grade markings, measure the shaft diameter and count the threads.
Can I use a metric caliper to measure an imperial fastener?
Yes. A metric caliper will give you the measurement in millimeters, which you can convert to inches by dividing by 25.4. For example, 6.35 millimeters is 1/4 inch. However, it is easier to use a caliper that displays both units, or to measure in the system that matches the fastener.