A US business card trims at 3.5 by 2 inches. That is the number everybody knows and it is not the number that ruins cards. The two that do are the bleed and the safe margin. Bleed is the 0.125 inches of artwork you extend past the trim on every edge, so a stack of cards guillotined a hair off centre still has ink to the edge instead of a white hairline down one side; supply it and your file is 3.75 by 2.25 inches, not 3.5 by 2. The safe margin is the 0.125 inches inside the trim you keep text out of, for the same reason in reverse. Two commercial printers publish exactly these numbers and agree on all three. What is left after both is a live area of 3.25 by 1.75 inches — 234 by 126 points, 29484 square points. That is the entire budget the card has, and it is about a twentieth of a letter page.
Because the live area is a number, so is everything you can put in it. This builder sets each field at a fixed type size — the company name at 13 point, contact lines at 8, the licence line at 7 — and divides the column width by half the type size, the standard prepress figure for average glyph width in a Helvetica-class face. A 13 point company name in the 234 point column gets 36 characters. Choose the accent-band layout and the bar takes 36 points off the left, leaving a 198 point column, and the same name gets 30. The budget moves when the layout moves because both are computed from the same geometry. Nothing here is a rule of thumb, and nothing is a warning you can only get by exporting, printing and looking.
The vertical direction is the one that catches people, because a screen has no bottom edge. Lines flow down from the top of the live area at the leading the exporter writes, and the licence line is pinned to the bottom of it rather than added to the flow — a licence number must not be the thing that gets pushed off the card. When the flowed copy reaches the pinned line, that is a collision, and it is reported in points against the 2 inch card height. Add a service area, a job title and a second phone number and you will find the ceiling faster than you expect. The honest fix is fewer lines, not smaller type: a card set at 5 point is a card nobody reads in a dim garage.
The legal part is the one no competitor card template mentions, and it is worth knowing precisely rather than approximately. Several states require a contractor's licence number on all advertising. Two of the rules verified here name business cards, and they get there differently. In Texas, 16 Texas Administrative Code section 73.51(e) requires an electrical contracting company's soliciting advertising to carry the contractor's name and licence number, and the rule text then says, in so many words, that this includes business cards. In California, Business and Professions Code section 7030.5 says only "all forms of advertising" — but the registrar's own rule defining that term, 16 California Code of Regulations section 861, opens its list with "any card". Florida is the instructive contrast: the word "card" does not appear anywhere in section 489.119. Its subsection (5)(b) reaches "each offer of services, business proposal, bid, contract, or advertisement, regardless of medium", which a card plainly is — but by category, not by name. This builder grades those three and says which kind of coverage each one is. For every other state it prints "not evaluated — check your state board", because there is no authoritative fifty-state table to copy and inventing one would put a wrong verdict on real advertising.
The last thing the builder does is give you a sheet you can actually print. A US Letter page is 8.5 by 11 inches; hold 0.25 inches off every edge for the non-printable margin most desktop printers impose and you have 8 by 10.5 inches of printable area. Upright cards at 3.5 by 2 give two columns and five rows — 10 cards a sheet. Rotating them ninety degrees would fit twelve, and that is deliberately not what this exports: no perforated letter business-card stock is made in that arrangement, whereas the 2 by 5 upright grid is exactly the layout Avery's 5371 stock publishes. Centring it leaves 0.75 inch side margins and 0.5 inch top margins, both comfortably outside the printable edge. The trim-size pages are the ones you send to a press; the gang-up sheet is the one you run yourself the night before a trade show.
An HVAC card gets left on a furnace, taped inside an air handler door, and stuck to a fridge. It is read three years later by someone who has forgotten who installed the system, so the company name and the phone number have to survive at arm's length — which is an argument for fewer lines, not smaller type.