10 Centimeters to Inches

Ten centimeters is 3.9370 inches, and one decimeter

Ten centimeters is 3.9370 inches. Divide 10 by 2.54 and the digits run 3.937007874, so the answer is exact as far as you care to write it down. Ten centimeters is also 100 millimeters and 0.1 meter, which makes it the one length in this family that lands on a metric round number: a decimeter.

The NIST Metric Program keeps the US definition of the inch at 25.4 mm, and the BIPM SI Brochure defines the meter that the centimeter divides. Both are fixed definitions rather than measurements, so 10 cm is 3.9370 in in any year and any country. The object being measured never changes the factor.

In fractions the value is 3 15/16 in, which is 3.9375 in and five ten-thousandths of an inch above the decimal figure. Four inches is 10.16 cm. That 1.6 mm gap between a round inch figure and a round metric figure is the whole story of this page.

Four inches is 10.16 centimeters, so the shortcut runs 1.6 mm long

Multiply 10 by 0.4 and you get 4.0, the number most people carry in their head for this conversion. The exact answer is 3.9370, so the shortcut runs 0.063 in high, or 1.6 mm. On one measurement nobody notices. On a layout the error compounds with every repeat, because each piece is cut a hair long.

Worked example: a tile grid. Ten tiles of 10 cm laid in a row cover 100 cm. Ten tiles of 4 in cover 40 in, which is 101.6 cm. The two rows finish 1.6 cm apart, a bit over half an inch, and that is the width of a grout line plus a little. The same arithmetic decides whether a 10 cm accessory drops into a 4 in opening.

The correction is easy to carry. Subtract 0.063 in for every ten centimeters, which is 1.6 mm, from the shortcut result. At 30 cm that turns 12.0 into 11.81. At 100 cm it turns 40.0 into 39.37. For a dough thickness or a yarn length the shortcut is fine. For a slot that has to swallow a filter, use the exact figure.

Prints and frames: 4 by 6 inches against 10 by 15 centimeters

A 4 by 6 inch print is 10.16 by 15.24 cm. A print sold as 10 by 15 cm is 3.937 by 5.906 in. The two sizes sit close enough that shops mix them up and far enough apart that a mat cut for one will not sit square on the other.

Worked example: a 10 by 15 cm photo in a 4 by 6 frame. The frame opening leaves 1.6 mm of play on the short side and 2.4 mm on the long side. A mat with a 3.5 by 5.5 in window, which is 8.89 by 13.97 cm, covers 1.1 cm of the short edge and about 1.0 cm of the long edge. That is why metric prints so often arrive with a white border built in.

Paper runs on the same two rails. A4 is 21 by 29.7 cm, or 8.27 by 11.69 in, and US Letter is 8.5 by 11 in. A4 is 0.23 in narrower and 0.69 in longer, which is why a document laid out for one system always shows a shift on the other, and why the ten centimeter margin you set in a metric template reads 3.94 in on a US printer dialog.

Nursery pots: a four inch pot is 10.16 cm across the rim

Garden centers in the US size pots by top diameter in inches, and the metric figure on the tag is where ten centimeters turns up. A 4 in pot is 10.16 cm across, a 3 in pot is 7.62 cm, and the 6.5 in pot sold as a gallon is 16.51 cm.

Worked example: a 10 cm pot beside a 4 in pot. The diameters differ by 1.6 mm, and volume falls with the cube of the diameter, so the 10 cm pot holds about 4.6% less soil mix. Across 500 pots at half a quart each, that is about 3 gallons of mix, which changes no planting plan and does change a wholesale order.

Depth matters as much as width, and it is where the naming gets loose. A 4 in pot is usually 4 to 5 in deep, while a 10 cm pot may be 8 or 9 cm deep, so the two hold different root balls even when the rims match. If a supplier quotes a 10 cm pot, ask for the depth before you compare it with a US size.

Seed trays follow the same logic. A 10 by 10 cm cell block is 3.937 by 3.937 in, and a 4 by 4 in block is 10.16 by 10.16 cm. Fifteen of the metric blocks fill a 50 by 30 cm tray with no waste, while fifteen of the inch blocks need a tray 30.48 by 50.8 cm.

Sewing patterns: ten centimeters is 3 15/16 inches

Patterns printed in Europe list every measurement in centimeters, and a US sewist reading one needs an inch figure at each seam. Ten centimeters is 3.937 in. The standard 5/8 in seam allowance is 1.5875 cm, which is why metric patterns round it to 1.5 cm and lose 0.9 mm per seam.

Worked example: a hem folded twice at 10 cm. The first fold takes 10 cm and the second takes another 10 cm, so the fabric used is 20 cm, or 7.874 in. Cut the same hem at 4 in per fold and you use 8 in of cloth, 0.126 in more, which across six panels is three quarters of an inch of extra fabric.

Fabric width is the other place the two systems meet. A bolt of 150 cm is 59.06 in and sells as 60 in. A bolt of 112 cm is 44.09 in and sells as 44/45 in. A 10 cm strip cut from the 150 cm bolt is 3.937 in wide, and a 4 in strip is 10.16 cm, so a pattern that asks for eight 4 in strips needs 81.28 cm of the bolt, not 80 cm.

Buttons and trims carry the same split. A 10 mm button is 0.39 in, a 15 mm button is 0.59 in, and the 5/8 in button sold in US stores is 15.875 mm, which is close enough for a buttonhole but not for a shank.

Ten centimeters of rain is 3.94 inches

A metric rain gauge reads in millimeters, and 10 cm on that gauge is 100 mm, which is 3.937 in. A US gauge calls the same storm a four inch rain. The gap between 3.94 and 4.00 in says nothing about the weather and plenty about the runoff math.

Worked example: 10 cm of rain on a 1,000 sq ft roof. The area is 92.9 m squared, the depth is 0.1 m, so the volume is 9.29 cubic meters, or 9,290 liters, which is 2,454 gallons. A 1,500 sq ft roof collects 3,681 gallons from the same storm, and a 2,000 sq ft roof collects 4,908.

The National Oceanic and Atmospheric Administration publishes storm totals in inches for the US and in millimeters for its international partners, so both figures turn up in the same report. Snow is a separate case. Ten centimeters of snow usually melts to about 1 cm of water, or 0.39 in, though the ratio runs from 5 to 1 in wet snow to 20 to 1 in dry powder, so the depth on the ground is a poor guide to the water in it.

For a garden, the useful figure is depth rather than volume: 10 cm of rain wets the top 10 cm of soil, which is 3.94 in, and that is more than most vegetable beds need in a week. A 2.54 cm rain, one inch, is the amount a sprinkler run is usually scheduled to replace.

Ten centimeters beside the numbers around it

The short ladder around ten centimeters is worth memorizing, because most everyday estimates land on it. 2.54 cm is exactly 1 in, 5 cm is 1.969 in, 7.62 cm is 3.00 in, 10 cm is 3.937 in, 12 cm is 4.724 in, and 25.4 cm is 10.00 in.

Objects help fix the size. A US dollar bill is 6.14 by 2.61 in, which is 15.6 by 6.6 cm, so ten centimeters covers a little under two thirds of its long edge. A 3 by 5 in index card is 7.62 by 12.7 cm. A credit card is 3.37 by 2.13 in, or 8.56 by 5.40 cm. None of these is a ruler, and each one catches a number that is off by a whole inch before it reaches an order form.