Measure garden tap size by checking thread diameter across the spout and matching it to BSP or GHT charts.
Getting the right connector starts with knowing the thread on your outdoor faucet. A quick check with a ruler or calipers tells you the outside diameter. From that number, you can map it to common standards such as BSP for the UK and much of Europe, or GHT for North America. This guide shows the fast way to confirm the size, avoid leaks, and pick parts that fit the first time.
Measure A Garden Tap Thread (Quick Steps)
- Wrap tape around the spout thread and mark one full circle. Remove and lay it flat to read the circumference, then divide by 3.14 to estimate the diameter.
- Or place calipers across the outer peaks of the thread and read the outside diameter in millimetres.
- Count the number of thread peaks along a 1" span (or measure pitch in mm). This helps confirm BSP vs GHT or other types.
- Match your numbers to the chart below. Pick the closest standard size rather than the exact reading on your ruler.
Thread Reference Table (Fast Match)
The figures below are the outside diameters you see on the male spout, not the pipe bore. Pick the nearest match.
| Thread Standard | Nominal Size | Outside Diameter (mm) |
|---|---|---|
| BSPP/BSPT | 1/2" BSP | ~20.96 |
| BSPP/BSPT | 3/4" BSP | ~26.44 |
| BSPP/BSPT | 1" BSP | ~33.25 |
| GHT (NH) | 3/4" GHT | ~26.95–27.0 |
| Metric Aerator | M22 | ~22 |
| Metric Aerator | M24 | ~24 |
Why Your Ruler Reading Looks “Off”
A label such as 1/2" BSP refers to a historic pipe size, not the visible diameter on the spout. That is why a 1/2" BSP thread measures close to 21 mm across the peaks, and a 3/4" BSP lands near 26.4 mm. The naming comes from old pipe bores, so always trust the charted outside diameters, not the name alone.
Confirm The Standard Before You Buy
Most outdoor taps in the UK use 3/4" BSP male threads, while North American hose bibs use a 3/4" GHT (also called NH) form with 11.5 threads per inch. A BSP tap connector is the common mate in the UK. The numbers are close, but the thread angle and pitch differ, which can cause drips or cross-threading. If you live in a mixed-standard setting, check both diameter and pitch.
Tools That Make Sizing Easy
- Steel ruler or calipers: read outside diameter across thread peaks.
- Thread gauge: count threads per inch or pitch in mm to separate GHT, BSP, NPT, and metric.
- Reference card: print a 100% scale thread card for aerators and small fittings, then place your part on the outline.
- Phone photo with scale: snap a straight-on shot with a coin or tape beside the spout to compare later.
Pick The Right Connector For Outdoor Use
Once you know the thread, choosing an adapter is simple. In the UK, many quick-connect systems include combined female fittings that screw onto 1/2", 3/4", or even 1" BSP. In North America, look for parts marked 3/4" GHT or 3/4" NH. Avoid mixing types unless the adapter is designed for that job.
Field Test: Does The Part Seat And Seal?
Even with the right size, a poor seal can leak. Hand-tighten the connector until the washer compresses. If you see wobble, back off and start square. Replace tired washers, use PTFE tape on tapered threads where the standard calls for it, and keep grit off the sealing face.
Where The Standards Differ
BSP threads use a 55° profile and come in parallel (BSPP) and tapered (BSPT) forms. GHT uses a 60° profile with a fixed pitch of 11.5 TPI at 3/4". NPT also uses 60° and is tapered with pitches that vary by size. These details explain why two parts that look the same can feel tight yet still seep under pressure.
Spotting A Bathroom Tap Aerator Size
If you are measuring a kitchen or basin outlet rather than an outdoor spout, you might be dealing with metric aerators. Common sizes are M22 and M24. Remove the insert and measure the thread diameter on the housing. Printed sizing cards let you lay the part on a life-size outline to match it in seconds.
Real-World Sizes You Will See
Here are the readings that most users find on the first measurement. If yours sits between values, read the pitch and check the second table later in this guide for adapter picks.
| Common Scenario | Part You Need | Notes |
|---|---|---|
| UK outdoor spout ~26.4 mm across | 3/4" BSP female hose connector | Standard on most garden fittings |
| UK outdoor spout ~20.9 mm across | 1/2" BSP female hose connector | Seen on older or compact taps |
| North America hose bib ~27 mm, 11.5 TPI | 3/4" GHT female hose end | Also labelled NH |
| Kitchen outlet with removable aerator ~24 mm | M24 aerator adapter to quick-connect | Match male vs female |
| Kitchen outlet ~22 mm | M22 aerator adapter to quick-connect | Match male vs female |
| Legacy hardware with NPT | NPT adapter to BSP or GHT | Confirm tapered thread |
Step-By-Step: Map Your Numbers To A Size
1) Read Outside Diameter
Measure across the outer thread peaks. If the reading is near 26.4 mm, you are in 3/4" BSP territory. Near 21 mm hints at 1/2" BSP. Around 33 mm points to 1" BSP. A reading near 27 mm with 11.5 TPI aligns with 3/4" GHT.
2) Check Pitch Or TPI
Lay a 1" scale along the thread and count peaks. Eleven and a half per inch is the garden hose form used in the US and Canada. Fourteen per inch with a 55° form signals BSP at 1/2" or 3/4" sizes. Metric aerators list pitch by thread designation rather than TPI.
3) Decide Parallel Or Tapered
Parallel threads seal with a washer or O-ring at a flat face. Tapered threads seal in the threads and may need PTFE tape. Mix-and-match seals only when a maker explicitly states the parts are cross-standard compatible.
Buying Tips That Save A Return Trip
- Pick a combo female connector set that covers 1/2", 3/4", and 1" BSP if you service many taps.
- Carry a short thread gauge strip in your wallet or toolbox. A cheap plastic one is fine.
- Stick to branded hose ends for everyday use; they hold washers in place and thread cleanly.
- Add spare washers to the kit. Most drips come from a tired seal, not the thread.
Troubleshooting Fit And Leaks
Cross-Threading
If the part binds early, stop and start square. A mismatch between BSP and GHT can feel fine for a turn or two, then jam. Back off and confirm the pitch.
Washer Issues
Flat washers harden with age. Swap the washer before you hunt for a new connector. A new seal often cures spray at the joint.
Taper Vs Parallel Mix
Do not force tapered into parallel. The first threads may grab, yet the seal sits in the wrong place. Match like to like or use the right adapter.
When You Should Use An Adapter
Adapters are handy when fixtures from different regions meet. Use a labelled BSP-to-GHT ring for travel kits or imported gear. Keep threads clean and do not over-tighten. If a fix feels shaky, choose a one-piece connector in the target standard.
Safe Link Picks For Reference
Compare your diameter with a BSP size chart. For North American hose ends, parts are marked 3/4" GHT or 3/4" NH with 11.5 TPI.
Regional Notes And Edge Cases
Mixed stock appears in border regions and on imported fixtures. You might see a UK yard hydrant with a US hose end fitted by a previous owner. In that case, measure both pieces. If the spout is BSP and the hose end is GHT, a short adapter keeps wear off the original thread.
A handful of older taps carry 5/8" BSP or unusual forms. When your reading lands near 23 mm but the pitch does not match the common lists, take a quick photo, count the peaks, and bring both numbers to the shop. Staff can match by gauge in minutes.
Care And Seasonal Storage
Thread wear often comes from grit and side-load. Keep hose weight off the spout with a wall hook or reel. Before winter, back off the connector, dry the washer, and store the hose indoors. A light coat of silicone grease on the washer keeps it supple and makes spring hookups smooth.
Summary Checklist You Can Screenshot
- Measure across the peaks, not the valleys.
- Read to the nearest millimetre.
- Count TPI or read pitch.
- Map to the charted size, not the pipe name.
- Choose an adapter only when standards differ.
- Tighten by hand until the washer seats; no pliers on soft brass.