International bittering units (IBUs) are used to tell how bitter your beer is (higher value means more bitterness). The IBU scale starts at zero for beers with no bitterness (fruit beers) and goes up to 120+ for the super bitter and hop rich beers like Imperial IPA and American Barley Wine. The following equations assume the Tinseth bittering scale.
To calculate IBUs we apply factors for the Alpha Acid Units (AAU), recipe volume (Vol), boil gravity and the boil time.
Calculating AAU:
• AAU = Weight of hops (oz) x % Alpha Acid rating of the hops.
• When hops are measured in grams, divide grams by 28 to get ounces.
• Example 1: 1.5 oz Brewer's Gold, having 6.7% Alpha Acid rating = 1.5 x 6.7 = 10.05 AAU.
• Example 2: 42 g Brewer's Gold, having 6.7% Alpha Acid rating = (42 / 28) x 6.7 = 10.05 AAU.
Calculating Bitterness:
The equation for IBUs is:
• IBU = AAU x U x 75 / Vol
The time and gravity factor of the boil are expressed as the utilization (U). 75 is a constant for the conversion of English units to Metric. The proper units for IBUs are milligrams per liter, so to convert from ounces per gallon a conversion factor of 75 (74.89) is needed.
Gravity of the Boil:
• See Calculating Gravity for details.
• The gravity of the boil is used for figuring the utilization (U).
Utilization:
This number describes the efficiency of the isomerization of the alpha acids as a function of time and boil gravity. The efficiency decreases as gravity increases. The utilization numbers of the Tinseth scale are shown in the Gravity vs. Time table below.
Utilization as a function of Boil Gravity and Time (Tinseth):
| Gravity vs. Time | 1.030 | 1.040 | 1.050 | 1.060 | 1.070 | 1.080 | 1.090 | 1.100 | 1.110 | 1.120 |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 |
| 5 | 0.055 | 0.050 | 0.046 | 0.042 | 0.038 | 0.035 | 0.032 | 0.029 | 0.027 | 0.025 |
| 10 | 0.100 | 0.091 | 0.084 | 0.076 | 0.070 | 0.064 | 0.058 | 0.053 | 0.049 | 0.045 |
| 15 | 0.137 | 0.125 | 0.114 | 0.105 | 0.096 | 0.087 | 0.080 | 0.073 | 0.067 | 0.061 |
| 20 | 0.167 | 0.153 | 0.140 | 0.128 | 0.117 | 0.107 | 0.098 | 0.089 | 0.081 | 0.074 |
| 25 | 0.192 | 0.175 | 0.160 | 0.147 | 0.134 | 0.122 | 0.112 | 0.102 | 0.094 | 0.085 |
| 30 | 0.212 | 0.194 | 0.177 | 0.162 | 0.148 | 0.135 | 0.124 | 0.113 | 0.103 | 0.094 |
| 35 | 0.229 | 0.209 | 0.191 | 0.175 | 0.160 | 0.146 | 0.133 | 0.122 | 0.111 | 0.102 |
| 40 | 0.242 | 0.221 | 0.202 | 0.185 | 0.169 | 0.155 | 0.141 | 0.129 | 0.118 | 0.108 |
| 45 | 0.253 | 0.232 | 0.212 | 0.194 | 0.177 | 0.162 | 0.148 | 0.135 | 0.123 | 0.113 |
| 50 | 0.263 | 0.240 | 0.219 | 0.200 | 0.183 | 0.168 | 0.153 | 0.140 | 0.128 | 0.117 |
| 55 | 0.270 | 0.247 | 0.226 | 0.206 | 0.188 | 0.172 | 0.157 | 0.144 | 0.132 | 0.120 |
| 60 | 0.276 | 0.252 | 0.231 | 0.211 | 0.193 | 0.176 | 0.161 | 0.147 | 0.135 | 0.123 |
| 70 | 0.285 | 0.261 | 0.238 | 0.218 | 0.199 | 0.182 | 0.166 | 0.152 | 0.139 | 0.127 |
| 80 | 0.291 | 0.266 | 0.243 | 0.222 | 0.203 | 0.186 | 0.170 | 0.155 | 0.142 | 0.130 |
| 90 | 0.295 | 0.270 | 0.247 | 0.226 | 0.206 | 0.188 | 0.172 | 0.157 | 0.144 | 0.132 |
Handy formulas for 1 gallon batches with hops measured in grams:
• IBU = (grams of hops / 28) x Alpha Acid % x U x 75
For larger batches, divide the "IBU" factor by the batch volume.
• X grams hops = (((desired IBU / 75) / U) / Alpha Acid %) x 28
For larger batches, multiply the "X grams hops" factor by the batch volume.