4 centimeters is 1.5748 inches, just past the 1 9/16 mark

Four centimeters equals 1.5748 inches. The full quotient is 1.5748031496 in, from 4 / 2.54. A tape marked in sixteenths carries that length 0.0123 in beyond the 1 9/16 line, and a metric ruler shows the same distance as 40 mm.

NIST Special Publication 811, Appendix B prints 1 in = 2.54 cm as an exact factor (NIST, 2008), and the BIPM SI Brochure ties the inch to the metre at 0.0254 m (BIPM, 9th edition, 2019). Both sources treat the number as a definition, not a measurement, so the digits past 1.5748 are yours to round.

The last three rows run the same number through the yard and the foot. Four centimeters is 0.04 m, which is 0.1312 ft, which is 0.0437 yd. For a part that small, only the inch and millimeter rows get used in practice.

Finding 1.5748 on a tape, mark by mark

US tapes carry eighths, sixteenths and thirty-seconds, and 40 mm lands between two lines on each of those scales. The table lists the neighbors in millimeters, so the gap against 4 cm is visible without doing the division twice.

Two multiplication steps explain the rows. Multiply 1.5748 by 16 and you get 25.20, so the sixteenths scale splits 40 mm between 25/16 in at 39.69 mm and 26/16 in at 41.28 mm. Multiply by 64 instead and the count is 100.79, which puts 40 mm a whisker under 101/64 in at 40.08 mm.

Which mark to use depends on the fit. A trim piece cut at 1 9/16 in runs 0.31 mm small, close enough that paint hides it. Anything that has to slip over a 40 mm shaft needs the larger side, so 1 5/8 in or a reamed 40 mm bore.

A 40 mm watch case, a silver dollar and two screw lengths

Sizes in this range get described in millimeters by one trade and in inches by another, and 4 cm sits right where the two vocabularies collide.

A 40 mm watch is the same width as 1.5748 in, and 38 mm cases sold as 1.5 in are 1.9 mm smaller across. Watch listings quote the case diameter in millimeters, so the inch number is a conversion rather than a size anyone measures.

Screws work the other way. A US hardware bin holds #8 x 1 1/2 in, which is 38.1 mm long, and #8 x 1 5/8 in, which is 41.3 mm. A metric drawing that calls for a 40 mm screw sits between the two bins, 1.9 mm longer than the first and 1.3 mm shorter than the second. Dropped through a 35 mm deep hole, the 40 mm screw leaves 5 mm standing proud, which is room for a washer and a nut.

A 4 cm margin on paper, and the pixels it becomes

A 4 cm margin is a common setting on word processors outside the US, and it translates to 1.5748 in, which is wider than the 1 in default in Word.

US Letter is 8.5 by 11 in, or 215.9 by 279.4 mm. Set 40 mm on the left and right and the text column shrinks to 215.9 minus 80, which is 135.9 mm, or 5.35 in. Set the same 40 mm on all four sides and the live area is 135.9 by 199.4 mm. A 3 by 5 index card is 76.2 by 127 mm, so a 40 mm square pasted on it covers 16 cm2, roughly a sixth of the face.

Screens use a different unit again. CSS defines 1 in as 96 px, so 1 cm is 37.795 px and 40 mm is 151.2 px. A button styled at 4 cm is 151 px wide on a standard density display, which is wider than most phone toolbars.

Print shops in the US quote margins in inches, so a file built with 40 mm margins gets checked against 1.57 in. The 0.0048 in left over after 1.57 is 0.12 mm, under the width of the paper trim tolerance.

Squares, cubes and circles at 4 cm

Area and volume conversions catch people who convert the length and stop there. The factor gets squared or cubed along with the unit.

A 4 cm square is 16 cm2, and one square inch is 6.4516 cm2, so 16 / 6.4516 = 2.4800 in2. Scale that to a 4 cm cube and the volume is 64 cm3, which is 64 / 16.387064 = 3.9055 in3. The cube also holds 64 mL, or 2.16 US fluid ounces.

The circle numbers come from pi times the diameter and pi times the radius squared. Circumference is 3.14159 x 4 = 12.5664 cm, and area is 3.14159 x 2 x 2 = 12.5664 cm2, the same digits by coincidence of a diameter of 4. A 40 mm round coaster covers 1.9478 in2, a bit less than a 1.5 in square.

The mistake to avoid is multiplying an area by 2.54 instead of 6.4516, or a volume by 2.54 instead of 16.387. That inflates a 4 cm cube from 3.9 in3 to 10.2 in3.

Hoops, beads and cord sold at 40 mm

Jewelry listings mix millimeters and inches within the same shop, and a 40 mm measurement can arrive as 1.5 in, 1.57 in or 1 9/16 in depending on who wrote the listing.

A 40 mm hoop earring is 1.5748 in across, and the 1.5 in hoops sold in US stores are 38.1 mm, 1.9 mm smaller. On a 16 in chain, 406.4 mm of length, ten beads of 40 mm take 400 mm and leave 6.4 mm, which is enough for one small jump ring but not two. Swap to 38 mm beads and eleven fit at 418 mm, which overruns the strand.

Cord and wire follow the same math. A 4 cm extender chain adds 1.5748 in to a necklace, and three of them add 4.72 in. Beading wire sold as 0.4 mm is a different unit entirely, ten times thinner than 4 cm, so read the decimal point before ordering.

For a bracelet, a 40 mm inner width is 1.57 in, wide enough for a 1.5 in wrist band with 1.9 mm of clearance. That margin is thin, so most cuff patterns call for 42 mm, which is 1.65 in.

The 0.4 shortcut and the 0.64 mm it costs

Multiplying centimeters by 0.4 gives an answer fast. For 4 cm the shortcut returns 1.6 in, which is 0.0252 in high, or 0.64 mm. The error is 1.6 percent, small enough for a shopping decision and too large for a machined part.

The shortcut works because 0.4 is 0.3937 rounded up, so it always runs long, and the error grows with the number. At 40 cm the same rule gives 16 in against a true 15.748 in, a 6.4 mm overshoot. At 4 cm the difference hides inside a saw kerf.

Doubling is exact where the shortcut is not. If 4 cm is 1.5748 in, then 8 cm is 3.1496 in and 2 cm is 0.7874 in, with no rounding drift. That makes doubling the safer mental tool when the number is a multiple of 4.

For the reverse direction, inches to centimeters, multiply by 2.54. A 1.5 in bolt is 3.81 cm, and a 1.625 in bolt is 4.1275 cm, which brackets the 4 cm case from the table above.

Rain gauges, door gaps and other 40 mm clearances

Weather reports in metric countries give rain in millimeters, and 40 mm of rain is 1.5748 in. That is a heavy two-day storm rather than a passing shower, and the volume it drops is where the conversion earns its keep.

One millimeter of rain over one square meter is one liter. Forty millimeters over a 1,000 sq ft roof, which is 92.9 m2, is 40 x 92.9 = 3,716 L, or 981.7 US gallons. In inches the same storm is 1.5748 in over 1,000 sq ft, and the familiar rule of 0.62 gallons per square foot per inch returns 976 gallons before rounding. The two routes agree within half a percent.

Around the house, 4 cm is a common gap spec. A door undercut 40 mm clears a 1.57 in threshold, and a 40 mm air gap behind a cabinet is 1.5748 in of clearance for a plug. A 40 mm hole saw is not a stock US size, so the closest cutter is 1 5/8 in at 41.28 mm, which leaves 0.64 mm of slop on each side of the cut.

Tile and flooring spacers get specified in millimeters for the same reason: 4 mm and 40 mm differ by a factor of ten, and reading one as the other turns a 1/8 in grout line into a 1.57 in gap.