page updated 2012-10-20

W&B KG to LB Conversion

2.20462262185 lb/kg 0.45359237 kg/lb

The W&B program holds and calculates weights in pounds. However, weights can be entered in pounds or kilos. They're always displayed as whole pounds or whole kilos. If you enter in kilos but choose to display in pounds, you may see an apparent discrepancy in that if you add up the displayed pounds, the added total may not exactlay equal the displayed total.

For example, suppose you enter a one kilo load in each of five positions. Weights are held as single-precision floating-point binary numbers, in other words equivalent to seven significant decimal digits. So internally each of those five positions would be held as 2.204623 lb. Each would be displayed as 2 lb. When the program adds the internal representation of the weights, their sum would be 5 x 2.204623 = 11.02312, and that sum would be rounded to the nearest whole pound and displayed as 11. Adding the 5 rounded displayed values gives 10.

Boeing says it like this:

“When totals or summations are required the English values are summed separately from the metric values, dfferences may occur when comparing the English total with the metric total due to round off.” — Weight and Balance Control and Loading Manual, 1-00-001, Page 6 of 6, Jul 29/2003, D043U542-SIA1

Historical Note

About 20 years ago, the one airline using the W&B program at the time signed a contract with a foreign carrier to supplement their freight capacity. The contract specified that each 747 would carry a mimimum number of kilograms of freight. The contract also required that the conversion factor to be used would be 2.2 lb/kg, a commonly accepted value for conversion at the time. I demurred, pointing out that if you loaded 240,000 lbs of cargo, with that figure having been arrived at using a 2.2 conversion factor, and then fueled to the airplane to it's maximum takeoff weight, you would be taking off in excess of 1,000 lbs over gross.

Now a 747 isn't going to have trouble leaving the runway a thousand pounds over max takeoff weight. Since I am no longer flying, I can now admit to having taken them into the air up to 30,000 lbs over gross on a few occassions, not intentionally though—at least on my part, well except for maybe once.

Though I wasn't willing to make the conversion factor 2.2, I let them talk me into making it variable. I made the default 2.20462262185 lb/kg, but an airline could optionally change it to any value between 2.2 and the real value, and they did.

I always regretted agreeing to that, and some time ago I took out the optional lb/kg conversion feature. 2.20462262185 lb/kg is the conversion factor I see on my HP calculator, it's the conversion factor I see in the Boeing manuals, and it's the value now hard-coded into the program.