How to Calculate ERA Step by Step

To calculate ERA by hand use the formula (Earned Runs × 9) ÷ Innings Pitched. The constant 9 exists because a regulation baseball game is nine innings, so the formula scales a pitcher's performance to a per-game rate. This is the official method under MLB Rule 9.16 (current edition), which defines earned runs and runs allowed, and Rule 9.02, which specifies how innings pitched are recorded.

The 9 standardises comparison between pitchers who throw different numbers of innings. Without it, a starter who allows 50 earned runs over 200 innings and a reliever who allows 20 over 50 innings could not be compared fairly. Multiplying by 9 and dividing by innings pitched converts both to a common unit: earned runs per nine innings.

Do not skip this step or substitute another number. Using 7 for softball games is a separate case, covered under that stat's own rules. For baseball, the constant is always 9.

Partial innings are recorded in thirds, a system that dates to the 19th century and is codified in MLB Rule 9.02. One out equals one-third of an inning, written as .1. Two outs equal two-thirds, written as .2. Three outs complete a full inning.

The most common error is treating 5.2 as the decimal 5.2. It is not. 5.2 innings means 5 and 2/3, which equals 5.6667. The correct decimal conversion table is:

• 1 out = 0.3333 innings (write as .1)
• 2 outs = 0.6667 innings (write as .2)
• 3 outs = 1.0 innings

Never use .33 or .67 in your record. The official notation uses .1 and .2. You convert to decimal only when you enter the calculation.

Take the earned runs the pitcher allowed. Multiply by 9. This gives a numerator that represents how many runs would be allowed if the pitcher threw a complete nine-inning game at the same rate.

Example: 42 earned runs × 9 = 378. This step is straightforward but critical to get right. Do not swap the order. The formula is (ER × 9) ÷ IP, not IP ÷ (ER × 9).

Divide the product from step 2 by the innings pitched, using the correct decimal conversion from step 1. Round the result to two decimal places. This is the ERA.

Example: 378 ÷ 162.6667 (not 162.2) = 2.3237, which rounds to 2.32. Using 162.2 yields 2.3305 and rounds to 2.33.Use the correct denominator every time.

A pitcher allows 25 earned runs in 180 complete innings. The innings are all whole, so no conversion is needed.

Step 2: 25 × 9 = 225. Step 3: 225 ÷ 180 = 1.25. ERA = 1.25. This performance sits well below the 2023 MLB average of 4.33, but it is also a small sample. Check reliability before drawing conclusions.

A pitcher allows 42 earned runs in 162 and 2/3 innings. Convert the partial: 2/3 = 0.6667, so innings pitched = 162.6667, not 162.2.

Step 2: 42 × 9 = 378.ERA = 2.32. This is very good in any modern context. The 2024 MLB average was 4.15 (Baseball-Reference).

A reliever enters with two runners on base, then allows one earned run over 2 and 1/3 innings. Innings pitched = 2.3333. Earned runs = 1 (the two inherited runners are charged to the reliever if they score after entry, per MLB Rule 9.16).

Step 2: 1 × 9 = 9. Step 3: 9 ÷ 2.3333 = 3.857. ERA = 3.86. This is unreliable. FanGraphs research states that ERA for relievers with fewer than 20 innings is not stable. For a reliever, use FIP or SIERA instead for a better projection.

To compute a season ERA from a game log, add all earned runs across all games, add all innings pitched across all games, then run the formula once. Never average individual game ERAs. Averaging ERAs gives incorrect weight to games with few innings.

Example: Game 1: 2 earned runs over 7 innings, ERA = 2.57. Game 2: 4 earned runs over 5 innings, ERA = 7.20. Average of ERAs = (2.57 + 7.20) ÷ 2 = 4.89. Correct season ERA: total ER = 6, total IP = 12, (6 × 9) ÷ 12 = 4.50.

Using 5.2 as a Decimal

Treating 5.2 innings as 5.2 instead of 5.3333 is the single most frequent error in this calculation. It produces an ERA that is too high by about 0.10 to 0.30 runs, depending on the total innings. Always convert .2 to 0.3333.

Confusing Earned Runs With Total Runs

ERA only counts earned runs. Unearned runs, those that score due to errors or passed balls, are excluded. The official scorer makes the judgment, following MLB Rule 9.16. Using total runs instead of earned runs inflates the ERA and misrepresents the pitcher's performance.

Reversing the Formula

The correct order is (ER × 9) ÷ IP. Reversing it to IP ÷ (ER × 9) produces a number that has no meaning as an ERA. Check your work by confirming the result is in the range of 1.00 to 10.00 for MLB pitchers. Outside that, you likely reversed it.

Rounding Too Early

Complete the division before rounding. Rounding the partial innings conversion or the intermediate product introduces error. Keep at least four decimal places until the final step.

Ignoring Inherited Runners for Relievers

A reliever who enters with runners on base is charged with those runners if they score. Failing to include them understates the ERA. The rule is the same across MLB, NCAA, and NFHS rulebooks.