How heavy should my balsa bridge be?
There's no universal number: the mass limit in your rulebook sets the ceiling. Reported bridge masses from 8 g to 230 g, and how to aim for the best ratio.
Updated · 1 min read
Short answer: as light as you can make it while it still carries the load, and always under your contest's mass limit. The right weight depends on the contest.
Examples from published sources
| Contest or source | Reported mass |
|---|---|
| Science Olympiad examples from a coach's blog | about 8 to 9.5 g |
| Lab prototype truss, 320 mm span | about 26 g |
| University outreach contest with 36 sticks of 1/8 in balsa | limit about 200 to 227 g |
The span, load and rules change everything, so compare yourself only with bridges from your own contest.
How to aim
- Know the mass limit and stay a margin below it, because scales differ.
- If the load is capped, the lightest bridge that reaches the cap wins, so build down toward the cap.
- If the load is uncapped, you trade mass for strength, so the best ratio is what counts. See the efficiency score.
- Weigh as you build, including glue, so there are no surprises on test day.