Although this subject is actually rather complicated, we will attempt to simplify it here.
Metric weight measurement is based on the 'International Prototype Kilogram' which is stored in a temperature controlled vault in France. All weights used for calibration purposes should be either directly or indirectly traceable to this standard weight. These calibration weights are themselves adjusted to various standards depending upon the accuracy required.

Accuracy standards in normal use are:

M2 - The required standard for trade sales by weight which is special in the fact that the weight value must range from EXACTLY the stated weight to PLUS a small tolerance amount.

M1 - The usual standard for industrial use.
F2 - A slightly higher standard not generally used.
F1 - The usual standard for precision balances for laboratory use.
E2 - The standard for very high accuracy laboratory balances.

These four weight standards allow a small tolerance of 'plus or minus' the exact weight, the actual tolerance value obviously reducing as the standard increases. Click Here for our 'Calibration and Certification' page which contains tables of weight accuracy and the standards required for the calibration of different types of scales.

1 Tonne = 1000 Kilograms
1 Kilogram = 1000 grams
1 Gram = 1000 milligrams

The following list shows some different (non-metric) common weight units in use, together with their relationship to the 'Gram'.

1 Pound (Lb.) =453.59237g.
1 Ounce (St.) =6350g.
1 Dram (d) =1.7718451g.
1 Troy Ounce (ozt) =31.1034768g.
1 Pennyweight (dwt) =1.55517384g.
1 Momme (mm) =37.509377g.
1 Grain (gr.) =0.064798g.
1 Carat (Ct.) =0.2g.
1 Newton (N) =9806.65g.

(There are many other weight units in use around the world not listed here.)