CAST Compliant Body Armour: What the UK Standard Means and How It Compares to NIJ
CAST compliant body armour has passed the UK Home Office's own test standard for bullet, knife or spike resistance. It's the benchmark UK police forces buy against. NIJ is the American equivalent, and the two aren't interchangeable: a vest rated under one standard tells you little about how it performs under the other.
What CAST is and why its standard matters in the UK
CAST is the Centre for Applied Science and Technology, the Home Office science unit that sets performance standards for police equipment, including body armour. It took over the role once held by the Home Office Scientific Development Branch, which is why older vests carry "HOSDB" ratings.
The current document is the Home Office Body Armour Standard 2017. It's written around UK threats: handguns and knives far more than rifles, because that's what UK officers meet on the street.
That focus matters. A standard built for American patrol work leans on handgun threats common in the US. The UK standard puts knife and spike testing on the same footing as ballistics. For background on how forces use these ratings in buying decisions, see our guide to body armour for UK police.
The CAST body armour levels: ballistic, knife and spike
The CAST standard rates armour in three separate families, and one vest can carry ratings from more than one. A "multi-threat" vest might read HO1 plus KR1 plus SP1: bullet, knife and spike protection in one panel.
| CAST level | Threat type | What it covers |
|---|---|---|
| HO1 | Handgun | 9 mm FMJ at a defined velocity |
| HO2 | Handgun | Higher-energy handgun threats than HO1; see the Home Office standard for test rounds |
| HO3 | Rifle | Rifle threats, needs hard plates; see the Home Office standard for test rounds |
| HO4 | Rifle | The most severe rifle threats, needs hard plates; see the Home Office standard for test rounds |
| KR1, KR2, KR3 | Knife | Rising knife resistance, tested at two energies, E1 and E2 |
| SP1, SP2, SP3 | Spike | Thin, pointed weapons such as screwdrivers, paired with the KR level |
The knife levels work differently from the bullet levels. A standardised blade is driven into the armour at a set energy, and it must not penetrate beyond a set depth, with the exact limits defined in the Home Office standard. A KR1 panel resists a 24 joule knife strike; KR2 covers a more forceful 33 joule strike at the cost of weight and stiffness.
KR1 suits most security and door work. Higher levels make sense only where the job carries a specific, known risk. Our guide to stab proof vests in the UK covers the knife and spike levels in more depth.
CAST vs NIJ: how the two standards line up
CAST and NIJ levels roughly correspond, but no official conversion exists between them. The table below is a practical guide for orientation, not a substitute for the actual rating.
| CAST level | Closest NIJ level (0101.06) | Closest NIJ level (0101.07) |
|---|---|---|
| HO1 | Around Level II | Around HG1 |
| HO2 | Around Level IIIA | Around HG2 |
| HO3 | Around Level III | Around RF1 or RF2 |
| HO4 | Around Level IV | Around RF3 |
| KR and SP | No ballistic equivalent; NIJ uses a separate stab standard | Same |
The mapping is never exact for 3 reasons:
- The two standards fire different rounds at different speeds.
- Shot counts, angles and placement on the panel differ between the tests.
- The rules on backface deformation, the dent the armour pushes into the body behind it, are measured differently.
So a Level IIIA vest isn't automatically HO2, and an HO2 vest isn't automatically IIIA. If the job needs one standard, the armour needs that rating. For the NIJ side, see the different levels of body armour and what Level 3A body armour stops.
Compliant, tested or listed: what the wording tells you
"CAST compliant" is a manufacturer's claim that a design meets the standard. On its own, the phrase proves nothing. What backs it is a test carried out on that exact model against the CAST protocol.
Three wordings turn up on product pages, and they don't mean the same thing:
- Tested to CAST: a lab tested the design using the CAST method.
- CAST compliant: the seller says the design meets the level. Ask what that's based on.
- Listed or approved: the design is accepted for police procurement, for example through the National Police Procurement Hub (NPPH) framework; check with the Home Office or NPPH for current approvals.
The weak spot is "designed to" or "meets CAST requirements" with no test reference. That wording often means the panel uses similar materials to a tested one. Similar materials aren't the same armour.
How to check a CAST compliant claim
Checking a CAST claim takes five minutes and a couple of emails. Run these steps before you pay:
- Ask which edition of the standard the armour was tested to. Ratings from older HOSDB documents aren't the same as 2017 ratings.
- Get the exact level in writing: HO, KR and SP, plus E1 and E2 where knife resistance is claimed.
- Request the test report or certificate with the lab name and a reference number.
- Check that the model name on the report matches the panel label.
- Confirm the report covers the panel itself, not only a carrier or cover.
Which standard should you look for?
The right standard depends on who you answer to. Use these rules:
- Working for a UK police force or contractor: look for CAST ratings, because procurement specs are written in CAST terms.
- UK security, door staff or private buyer worried about knives: look for KR1 with SP1, ideally CAST tested.
- Working abroad, or with an international organisation: NIJ ratings are often the requirement. Check the contract wording.
- Facing rifle threats: you need hard plates, rated HO3 or HO4, or NIJ Level III or IV.
Before travelling with armour or sending it overseas, read our page on whether body armour is legal in the UK and the rules for exporting it.
Ask for the test report before you pay
Pick one vest you're considering this week and email the seller three questions: which standard edition, which exact levels, and which test report number. A reputable supplier answers all three quickly. If the answers are vague, or the report names a different model, treat the "CAST compliant" label as marketing and keep looking.
