This Resource Centre provides general guidance only. Always follow the equipment manufacturer’s instructions, the product technical data sheet and your site’s safe system of work. Isolate stored energy before maintenance. For safety-critical, regulated or uncertain applications, ask a competent engineer or the product manufacturer to confirm the specification.
Metric, BSP and NPT: what is the difference?
These thread families may overlap in apparent diameter, but they use different dimensions and profiles.
Metric threads
Metric threads are described by nominal diameter and pitch in millimetres—for example, M10 × 1.5. They are commonly parallel, although metric taper forms also exist. In fluid connections, a metric parallel thread often works with a washer, O-ring or shaped sealing face rather than sealing on the thread itself.
BSP threads
British Standard Pipe threads are based on a nominal pipe size rather than the measured outside diameter.
- BSPP is parallel and is commonly designated with the letter G.
- BSPT is tapered and is commonly associated with R designations.
BSP threads use a 55° thread form. A BSPP connection normally requires a separate sealing feature, while a BSPT joint may seal on the tapered thread with a suitable approved sealant, depending on the design.
NPT threads
NPT is a tapered pipe thread commonly used in North American equipment. It uses a 60° thread form. Although some NPT and BSP sizes appear close, their pitch and thread form can differ. They should not be treated as interchangeable.
Why a near match is unsafe
A mismatched fitting may engage for a few turns and still have insufficient thread contact. Tightening it can split a port, strip the thread or leave a connection that fails under pressure, vibration or temperature cycling.
Match the exact standard and sealing method. If an adaptor is required, use a purpose-made, correctly rated component.
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