Using a metric or newton meter torque wrench requires a bit of math if you're trying to tighten a nut to a certain number of pound-feet. You'll have to do the conversion from English measurements (feet, inches and pounds) to metric units (meters, centimetres and kilograms or newton meters). Then you'll use the metric measurement as you turn the wrench or, if you're using a click-style torque wrench, set the target torque. A newton meter, like a pound-foot, is a measurement of force -- the word "meter" here refers to the unit of length, not to a dial with numbers on it like a water meter. A newton meter is a little less than 1.5 pound-feet.
Determine which measurement your torque wrench uses -- kilogram-metres, kilogram-centimetres or newton meters. It should be clearly labelled. The abbreviation for kilogram-metres is "kg.m"; "kg.cm" means kilogram-centimetres, and "NM" refers to newton meters. If your torque wrench isn't clearly labelled, replace it. Many digital torque wrenches can be set to read in either English or metric units.
Multiply the pound-feet of force you are seeking by 0.1383 if the wrench measures in kg.m. The resulting number is your target as you tighten the wrench. For example, 10 pound-feet times 0.1383 is 1.383 kg.m.
Multiply the pound-feet you are seeking by 1.356 if the torque wrench measures in NM and use that number as your target. For example, 10 pound-feet times 1.356 equals 13.56 NM.
If the wrench measures in kg.cm, convert pound-feet to kg.m, then multiply the kg.m by 100 to get kg.cm. For example, 10 pound-feet times 0.1383 equals 1.383 kg.m, which, when multiplied by 100, gives 138.3 kg.cm.
Tighten the nut, spark plug or other part you are trying to torque, using steady pressure, until the gauge on the torque wrench reaches the correct metric number. Do not jerk on the wrench. Your measurement will be inaccurate and you may damage the wrench.