Here's what I would do unless I had a special scale available.
Weigh yourself on a bathroom scale while holding the pumpkin. Then weigh yourself without it. Subtract! That'll give you the weight of the pumpkin.

George
Tahlequah, OK

Very clever! Provided He can hold it up while standing on the scale !(grin!).
How about puting a board on the scale and placing the pumpkin on it? Another is to guesstimate the volume of a sphere with 3 ft dia (1.5ft rad.). Assume a specific gravity of 1(as water) to calculate the weight.
Or, you can built an scale yourself. Take an 8 ft 2x4. Drill holes near two ends. Divide the space between thm into (say 10). Dril a hole at the nearest graduation on one end. Install a handle from that hole.
Hange uniform scale pans ( plywood_) from each end hole.
Place th pumpkinh on the short side and known weghts on the long side . Try lifting and adjusting the weigt until both sides are balanced. Say you had 20 lbs weight on the scale. So your pumpkin weighs 9X20 =180lbs:
It should be a fun project.

good luck holding that one up. Most bathroom scales wont go up as high as your pumpkin weighs. Here is an estimation chart that i use while growing. Seems pretty accurate. Using the chart i estimated mine at 210. Picked it last night and weighed it at 216. I just set it on the scale for a few seconds, the weight registered and i took it off the scale to see the reading.

Here is a link that might be useful: http://www.backyardgardener.com/weight.html

My bad. I thought it said "weighing of a 1 gram pumpkin".