How to Convert Between Fractional, Decimal, and American Odds

Different bookmakers display odds differently. A UK bookmaker might show fractional 5/2. A European bookmaker might display 3.50 decimal. An American sportsbook might show +250.

They're all the same odds, just expressed in different formats.

To compare odds across bookmakers and find the best price, you need to convert between formats. This guide shows you the formulas, worked examples, and a comprehensive reference table.

Fractional to Decimal Odds

Formula: Decimal = (Numerator / Denominator) + 1

This is the most common conversion you'll need.

Examples

5/2 fractional to decimal: (5 / 2) + 1 = 2.5 + 1 = 3.50

2/1 fractional to decimal: (2 / 1) + 1 = 2 + 1 = 3.00

11/8 fractional to decimal: (11 / 8) + 1 = 1.375 + 1 = 2.375

1/2 fractional to decimal: (1 / 2) + 1 = 0.5 + 1 = 1.50

1/4 fractional to decimal: (1 / 4) + 1 = 0.25 + 1 = 1.25

7/4 fractional to decimal: (7 / 4) + 1 = 1.75 + 1 = 2.75

The logic: fractional odds show profit only. You add 1 to include your stake in the total return.

Decimal to Fractional Odds

Formula: Fractional = (Decimal - 1) / 1, expressed as a ratio

This converts the other direction.

Examples

3.50 decimal to fractional: (3.50 - 1) = 2.50, or 5/2

3.00 decimal to fractional: (3.00 - 1) = 2.00, or 2/1

2.375 decimal to fractional: (2.375 - 1) = 1.375, or 11/8

1.50 decimal to fractional: (1.50 - 1) = 0.50, or 1/2

2.75 decimal to fractional: (2.75 - 1) = 1.75, or 7/4

1.25 decimal to fractional: (1.25 - 1) = 0.25, or 1/4

The logic: you subtract 1 from decimal to remove your stake, leaving profit only. Then express as a fraction.

Decimal to American Odds

For decimals 2.00 or higher: American = (Decimal - 1) × 100

For decimals below 2.00: American = -100 / (Decimal - 1)

American odds use plus or minus. Positive odds indicate underdogs, negative indicate favourites.

Examples (Underdog Odds, Positive American)

3.50 decimal to American: (3.50 - 1) × 100 = 2.50 × 100 = +250

4.00 decimal to American: (4.00 - 1) × 100 = 3 × 100 = +300

2.50 decimal to American: (2.50 - 1) × 100 = 1.50 × 100 = +150

2.20 decimal to American: (2.20 - 1) × 100 = 1.20 × 100 = +120

Examples (Favourite Odds, Negative American)

1.80 decimal to American: -100 / (1.80 - 1) = -100 / 0.80 = -125

1.50 decimal to American: -100 / (1.50 - 1) = -100 / 0.50 = -200

1.30 decimal to American: -100 / (1.30 - 1) = -100 / 0.30 = -333

1.91 decimal to American: -100 / (1.91 - 1) = -100 / 0.91 = -110

The logic: positive American odds show profit per 100 units staked. Negative American odds show stake needed to profit 100 units.

American to Decimal Odds

This converts from American back to decimal.

For positive American odds (underdogs): Decimal = (American / 100) + 1

For negative American odds (favourites): Decimal = (100 / |American|) + 1

Examples (Positive American)

+250 American to decimal: (250 / 100) + 1 = 2.50 + 1 = 3.50

+300 American to decimal: (300 / 100) + 1 = 3 + 1 = 4.00

+150 American to decimal: (150 / 100) + 1 = 1.50 + 1 = 2.50

+120 American to decimal: (120 / 100) + 1 = 1.20 + 1 = 2.20

Examples (Negative American)

-125 American to decimal: (100 / 125) + 1 = 0.80 + 1 = 1.80

-200 American to decimal: (100 / 200) + 1 = 0.50 + 1 = 1.50

-333 American to decimal: (100 / 333) + 1 = 0.30 + 1 = 1.30

-110 American to decimal: (100 / 110) + 1 = 0.909 + 1 = 1.909

Conversion Reference Table

Here's a comprehensive table showing common odds in all three formats plus implied probability. Use this for quick reference rather than calculating every conversion.

Decimal	Fractional	American	Implied Probability
1.25	1/4	-400	80.0%
1.30	3/10	-333	76.9%
1.33	1/3	-300	75.2%
1.40	2/5	-250	71.4%
1.50	1/2	-200	66.7%
1.67	2/3	-150	59.9%
1.80	4/5	-125	55.6%
1.91	10/11	-110	52.4%
2.00	1/1	+100	50.0%
2.10	11/10	+110	47.6%
2.20	6/5	+120	45.5%
2.50	3/2	+150	40.0%
3.00	2/1	+200	33.3%
3.50	5/2	+250	28.6%
4.00	3/1	+300	25.0%
5.00	4/1	+400	20.0%
6.00	5/1	+500	16.7%
7.00	6/1	+600	14.3%
10.00	9/1	+900	10.0%

Bookmark or screenshot this table. It's invaluable for quick conversions.

Quick Reference: Common Odds Conversions

Fractional	Decimal	American	Implied Probability
1/2	1.50	-200	66.7%
EVS (1/1)	2.00	+100	50.0%
6/4	2.50	+150	40.0%
2/1	3.00	+200	33.3%
3/1	4.00	+300	25.0%
5/1	6.00	+500	16.7%
10/1	11.00	+1000	9.1%

This simplified table covers the most common odds you'll encounter in football betting.

Converting Implied Probability

Once you have odds in a common format, you can calculate implied probability.

From decimal odds: Implied probability = 1 / Decimal

Example: 2.50 decimal Implied probability = 1 / 2.50 = 0.40 = 40%

From fractional odds: Implied probability = Denominator / (Numerator + Denominator)

Example: 5/2 fractional Implied probability = 2 / (5 + 2) = 2/7 = 28.6%

From positive American odds: Implied probability = 100 / (American + 100)

Example: +250 American Implied probability = 100 / (250 + 100) = 100/350 = 28.6%

From negative American odds: Implied probability = |American| / (|American| + 100)

Example: -110 American Implied probability = 110 / (110 + 100) = 110/210 = 52.4%

Notice: the conversions give the same implied probabilities regardless of which format you start from. That's correct. The odds are identical, just displayed differently.

Practical Scenario: Comparing Odds Across Bookmakers

Here's a real-world example of why conversion matters.

Team A to win. You're comparing three bookmakers:

Bookmaker A (Fractional): 5/2 Bookmaker B (Decimal): 3.40 Bookmaker C (American): +250

Convert all to decimal for comparison:

Bookmaker A: (5/2) + 1 = 3.50 Bookmaker B: 3.40 (already decimal) Bookmaker C: (250/100) + 1 = 3.50

So Bookmaker A and C offer 3.50, Bookmaker B offers 3.40.

Bookmaker A and C are better. The difference is small (0.10) but compounds. Over 100 £10 bets, that 0.10 difference is £100 total profit difference.

Without converting, you might have missed that Bookmaker B was worse.

Mental Shortcuts for Common Conversions

You don't always need exact calculations. Approximations work well:

2.0 decimal is roughly 1/1 fractional (evens)

3.0 decimal is roughly 2/1 fractional

1.5 decimal is roughly 1/2 fractional

4.0 decimal is roughly 3/1 fractional

5.0 decimal is roughly 4/1 fractional

For American odds:

+100 is roughly 2.0 decimal (near evens)

+200 is roughly 3.0 decimal

-100 is roughly 1.99 decimal (slight favourite)

-200 is roughly 1.50 decimal (stronger favourite)

These mental approximations let you quickly assess whether an odds price is reasonable, without needing exact calculation.

When Conversion Is Actually Necessary

Conversion is essential when:

1. Comparing odds across bookmakers that display different formats
2. Understanding what odds represent by calculating implied probability
3. Converting accumulators to total expected returns (usually decimal)
4. Calculating expected value (usually easier in decimal)

Conversion is not necessary when:

1. Betting at a single bookmaker that lets you toggle display format
2. You're familiar with one format and don't need to compare
3. A calculator is available (use OddsChecker, BetBrain, etc.)

Tools for Odds Conversion

If you don't want to calculate manually:

OddsChecker (https://www.oddschecker.com) shows multiple bookmakers' odds for major sports, automatically displaying odds comparisons.

BetBrain (https://www.betbrain.com) is similar, aggregating odds from multiple bookmakers.

Standalone Odds Calculators exist online. Google "odds converter" and multiple tools appear. These let you enter odds in one format and see all conversions instantly.

Spreadsheets can automate conversions. Set up cells with the formulas above and enter a starting odd. Get all conversions automatically.

For most bettors, using a bookmaker's toggle (decimal vs fractional display) is simplest. For serious betting, bookmarks or OddsChecker saves time.

Common Conversion Mistakes

Forgetting the +1 when converting fractional to decimal. The formula is (Num / Denom) + 1, not just (Num / Denom). Without the +1, you'll underestimate returns significantly.

Using the wrong formula for American odds below 2.0. Negative American odds require a different formula than positive. Remember: negative is for favourites (divide), positive is for underdogs (multiply).

Rounding too early. Keep full decimal places in intermediate calculations. Round only the final result. Rounding 2.374 to 2.37 won't matter, but rounding mid-calculation can compound errors.

Forgetting that American negative odds use absolute value. When dividing for negative American, use the absolute value. -110 becomes 110 in the denominator.

Assuming conversion is exact. Bookmakers sometimes adjust odds to avoid certain prices. A 2.50 decimal might convert to 5/2 fractional exactly, but some fractional odds don't convert cleanly. Bookmakers adjust fractional odds to whole numbers often.

In Summary

• Odds convert between formats using three main formulas.
• Fractional to decimal: Add 1 to (numerator / denominator).
• Decimal to fractional: Subtract 1, express as a ratio.
• Decimal to American: Multiply by 100 if above 2.0 (positive result).
• Divide -100 by (decimal - 1) if below 2.0 (negative result).
• The reference table above covers 20 common odds conversions across all three formats.
• Bookmark it.
• Conversion is essential for comparing odds across bookmakers and understanding implied probability.
• Most modern bookmakers let you toggle display format, making manual conversion less necessary.
• But understanding the conversion process ensures you're comparing odds correctly.
• Implied probability (calculated from odds using 1 / decimal for decimal odds) bridges odds to value.
• Compare your probability assessment to implied probability.
• This is how you find valuable bets.

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Frequently Asked Questions

Why do I need to convert odds if my bookmaker shows all three formats?

Most online bookmakers do show all three formats. But some don't, and if you're comparing across multiple bookmakers, they might display different formats. Conversion ensures you're comparing like with like. Also, understanding the mathematics behind conversion deepens your odds knowledge.

Is there a shortcut for converting odds without a calculator?

For approximations, yes. 2.0 decimal is roughly 1/1. 3.0 is roughly 2/1. But for precise conversions, especially when money is involved, use the formulas or a calculator.

Can I convert American odds directly to fractional without going through decimal?

Yes, but it's more complex. You'd need to convert American to decimal first, then decimal to fractional. It's easier to go through decimal as an intermediate step.

Why does the conversion table show such precise decimal odds like 1.91?

Bookmakers often use these precise odds to offer specific margins. 1.91 decimal represents 52.4% implied probability, which is clean for their models. You might see 1.909 or 1.91 depending on the bookmaker's precision settings.

If I always use one bookmaker, do I need to know conversions?

Not necessarily, if your bookmaker lets you toggle display format. But knowing conversion helps you understand what the odds represent and compare across bookmakers if you ever change.

Are there any conversions I should memorise?

Yes: 2.0 decimal = 1/1 fractional = +100 American = 50% probability. This is the central reference point. Others can be calculated or looked up.

Why is American odds +250 for a 28.6% outcome?

Because +250 means stake £100 to win £250 profit, giving £350 total return. 1 / 3.50 = 28.6%. The American format uses different numbers but represents the same probability as 3.50 decimal or 5/2 fractional.