Bondo Mixing Ratio Calculator
Calculate the exact amount of hardener needed for your Bondo body filler. Mixing ratio calculator for 30:1, 40:1, 50:1, and custom body filler mixes.
Filler & Hardener Details
Enter your filler amount and desired mix ratio to calculate the exact amount of hardener needed.
The total amount of Bondo body filler you are mixing.
Check your product’s technical data sheet for the manufacturer’s recommended ratio.
Mix Ratio Results
Exact measurements for your Bondo mixture
Enter your filler amount and desired mix ratio above, then click Calculate Mix Ratio to see the exact amount of hardener needed.
Common Bondo Mixing Ratios
Use this chart to quickly reference the recommended mixing ratios for various Bondo products and applications.
| Bondo Product | Mix Ratio (Filler : Hardener) | Hardener % | Notes / Application |
|---|---|---|---|
| Standard Body Filler | 30:1 to 50:1 | 2.0% – 3.3% | “Golf ball” to “pea” rule |
| Bondo Hair (Fiberglass) | 30:1 | 3.3% | Contains fiberglass strands for strength |
| Bondo Glass (Metal Reinforced) | 30:1 to 40:1 | 2.5% – 3.3% | Waterproof, hole filler, rust repair |
| Bondo Spot Putty | Ready-to-use | N/A | No hardener required, air-dry |
| Bondo Plastic Boner | 1:1 (by volume) | 50% | For flexible plastic bumper repairs |
Bondo Mixing FAQ
Everything you need to know about mixing Bondo body filler, adding hardener, and achieving the perfect cure.
The standard mixing ratio for most Bondo body fillers is between 30:1 and 50:1 (filler to hardener). A common rule of thumb is a “golf ball” size of filler to a “pea” size of hardener, which roughly equates to a 30:1 or 40:1 ratio.
For a 30:1 ratio, you need 1 part hardener for every 30 parts filler. For example, if you use 3 ounces of filler, you would add 0.1 ounces (or roughly 3 grams) of hardener. Always refer to the specific product’s technical data sheet for exact measurements.
Adding too much hardener (a richer mix, like 10:1 instead of 30:1) will cause the Bondo to cure very quickly, making it difficult to work with. It can also lead to excessive heat generation, bubbling, cracking, and a brittle finish that may not sand well.
Adding too little hardener (a lean mix, like 60:1 instead of 30:1) will prevent the Bondo from curing properly. It will remain tacky, soft, and gummy, making it impossible to sand or paint over. If this happens, you must scrape it off and start over.
At room temperature (70°F/21°C) and with the correct mixing ratio, Bondo body filler typically begins to set in 3 to 5 minutes and is ready to sand in 15 to 30 minutes. Curing times will vary based on ambient temperature and the exact amount of hardener used.