SettingsSettings for this calculationUS
The cross-sectional area in square inches.
This is the unit small structural areas are quoted in: a reinforcing bar's area, a bolt's tensile stress area, the net section of an angle, the contact area of a bearing plate. It is not the unit for floor or wall areas — those are square feet, and converting them through this page gives a number seven orders of magnitude away from anything useful.
Area
645.2 mm²
1 in = 25.4 mm exactly, so 1 in² = 25.4² = 645.16 mm². Exact by definition. Areas square the length error, so convert measured dimensions before squaring them, not after.
- Area entered
- 1 in²
- Square millimetres
- 1 in²
- Square centimetres
- 1 in²
They open the calculator with your figures already in it
Square Inches to Square Millimetres Calculator: 645 mm² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- NIST SP 811, Appendix B — 1 in² = 645.16 mm² (exact)
- International yard and pound agreement, 1959 — the inch is exactly 25.4 mm
Inputs used
- Area (in²)
- 1
Intermediate steps
- Area entered
- 1 in²
- Square millimetres
- 1 in²
- Square centimetres
- 1 in²
Confidence note: 1 in = 25.4 mm exactly, so 1 in² = 25.4² = 645.16 mm². Exact by definition. Areas square the length error, so convert measured dimensions before squaring them, not after.
Add the equipment this sizes
This result is a specification — 645.2 mm² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-12 · v1.0.0
Regulatory standards & verification citations2
- NIST SP 811, Appendix B — 1 in² = 645.16 mm² (exact)
- International yard and pound agreement, 1959 — the inch is exactly 25.4 mm
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Structural steel and reinforcement are the two trades where a cross-sectional area in the wrong unit does real damage, and they are also the two where the same building is routinely detailed in both. A rebar schedule from a US consultant lands on a site buying Canadian or European bar; a European section table is checked against an AISC capacity written per square inch. The conversion itself is a squared inch and nothing more, which is exactly why it deserves stating rather than assuming: the squaring is where people go wrong, halving or doubling a factor they remembered from lengths. The bar equivalence in the examples is here because it is the lookup this page will mostly be used for, and because the near-identity of a #5 and a 16M is a useful thing to have seen once.