Changing BB weight changes your fps reading, but not your gun’s power. Enter what you chrono’d and the weight you used, then pick a new BB weight to see the speed you can expect, plus a full table for every common weight.
How the BB weight conversion works
An airsoft gun gives each BB roughly the same amount of energy, measured in joules. Because energy depends on weight and the square of speed, a heavier BB must travel more slowly to carry the same energy.
This is why a gun that chronos at 350 fps on 0.20g is not "losing power" when it reads 313 fps on 0.25g. Use our fps to joules calculator to see the energy behind any reading.
BB weight conversion chart
What common chrono readings on 0.20g become with heavier BBs.
| On 0.20g | 0.23g | 0.25g | 0.28g | 0.30g | 0.32g | 0.36g | 0.40g | 0.45g | Joules |
|---|---|---|---|---|---|---|---|---|---|
| 280 fps | 261 | 250 | 237 | 229 | 221 | 209 | 198 | 187 | 0.73 J |
| 300 fps | 280 | 268 | 254 | 245 | 237 | 224 | 212 | 200 | 0.84 J |
| 320 fps | 298 | 286 | 270 | 261 | 253 | 239 | 226 | 213 | 0.95 J |
| 330 fps | 308 | 295 | 279 | 269 | 261 | 246 | 233 | 220 | 1.01 J |
| 350 fps | 326 | 313 | 296 | 286 | 277 | 261 | 247 | 233 | 1.14 J |
| 370 fps | 345 | 331 | 313 | 302 | 293 | 276 | 262 | 247 | 1.27 J |
| 400 fps | 373 | 358 | 338 | 327 | 316 | 298 | 283 | 267 | 1.49 J |
| 420 fps | 392 | 376 | 355 | 343 | 332 | 313 | 297 | 280 | 1.64 J |
| 450 fps | 420 | 402 | 380 | 367 | 356 | 335 | 318 | 300 | 1.88 J |
| 500 fps | 466 | 447 | 423 | 408 | 395 | 373 | 354 | 333 | 2.32 J |
Choosing the right BB weight
Heavier BBs are more stable in flight and less affected by wind, but they need more hop-up to fly flat and drop sooner if your gun doesn’t have the power to lift them. Matching the weight to your gun’s energy gives the best range and accuracy.
| Power on 0.20g | Joules | Suggested weight | Typical guns |
|---|---|---|---|
| Under 300 fps | Under 0.84 J | 0.20g | Gas and spring pistols |
| 300 to 330 fps | 0.84 to 1.01 J | 0.20g to 0.25g | Indoor and CQB AEGs |
| 330 to 360 fps | 1.01 to 1.20 J | 0.25g to 0.28g | Most AEGs and GBB rifles |
| 360 to 400 fps | 1.20 to 1.49 J | 0.28g to 0.30g | Upgraded rifles |
| 400 to 470 fps | 1.49 to 2.05 J | 0.30g to 0.36g | DMRs |
| Over 470 fps | Over 2.05 J | 0.36g to 0.45g | Bolt-action snipers |
Whatever you choose, re-tune your hop-up after changing weight. Our hop-up set-up guide shows how, and our BB calculator tells you how many BBs you get in a bag of each weight. For the range you can expect at each weight, try the Airsoft Range & BB Weight Calculator.
Frequently asked questions
What is 350 fps on 0.20g with 0.25g BBs?
About 313 fps. The energy stays at roughly 1.14 joules, so the heavier BB leaves the barrel more slowly. With 0.28g it would be about 296 fps, and with 0.30g about 286 fps.
Do heavier BBs lower your fps?
Yes. Your gun gives the BB roughly the same energy whatever its weight, so a heavier BB moves more slowly. That doesn’t make your gun weaker: heavier BBs hold their speed better, drift less in the wind and hit more consistently at range.
Do heavier BBs give more joules?
Often slightly. Many AEGs and springers transfer a little more energy to a heavier BB, so a real chrono reading can be a few fps higher than this calculator predicts. Treat the result as a guide and chrono with the weight your site uses.
What BB weight should I use?
As a rule of thumb: 0.25g for AEGs at about 330 to 350 fps, 0.28g to 0.30g for 370 to 400 fps, and 0.36g or heavier for DMRs and snipers. Lighter 0.20g BBs suit gas pistols and low-powered guns. Re-tune your hop-up whenever you change weight. Our BBs explained guide goes into more detail.
Should I chrono with 0.20g or the BBs I use?
Use whatever your site asks for. Most UK sites quote limits on 0.20g BBs, and some chrono with their own BBs. If your site uses a joule limit, the BB weight doesn’t matter as long as you convert correctly.
Stock up on BBs and gas
Quality BBs give consistent chrono readings and protect your hop-up and barrel.
More airsoft tools
Airsoft Range & BB Weight Calculator
Find the best BB weight for your gun’s power and how far it should reach with a good hop-up.
Open the calculator →FPS to Joules Calculator
Convert airsoft fps to joules (and back) for any BB weight, then check it against typical UK site limits.
Open the calculator →Airsoft BB Calculator
How many BBs are in a bag, how many mags it fills, how many game days it lasts and the cost per shot.
Open the calculator →Results are estimates for guidance. Always follow your site’s chrono and safety rules, and your gun maker’s advice.