Blood Alcohol Calculator UK

What Is Blood Alcohol Content (BAC)?

Blood alcohol content is a measure of the amount of alcohol present in your bloodstream, expressed as milligrams of alcohol per 100 millilitres of blood (mg/100ml). In the UK, it is the key measurement used by police and courts to determine whether a driver is over the legal drink driving limit.

Our blood alcohol calculator uses the Widmark formula — the internationally recognised scientific method for estimating BAC. It takes into account your gender, weight, what you drank, and how long ago you started drinking. While no online calculator can be 100% accurate due to individual biological differences, it provides a reliable general estimate.

If you have been charged with a drink driving offence, the court will use your official breath or blood reading — not an online estimate — to determine your drink driving sentence. Use our sentence calculator to see the likely penalties for your reading.

UK Drink Driving Limits

The legal drink driving limits differ between England, Wales and Northern Ireland on one hand, and Scotland on the other. Scotland lowered its limit in December 2014 under the Road Traffic Act 1988 (Prescribed Limit) (Scotland) Regulations 2014.

It is a criminal offence under Section 5(1)(a) of the Road Traffic Act 1988 to drive or attempt to drive with a blood alcohol level that exceeds the prescribed limit. A conviction will result in a DR10 endorsement on your licence, a driving ban of at least 12 months, and a criminal record.

Factors That Affect Your Blood Alcohol Level

Everyone processes alcohol differently. Two people who drink exactly the same amount can have very different BAC levels. Here are the key factors:

What Happens If You’re Over the Limit?

If you’re stopped by police and found to be over the drink driving limit, you face serious consequences. Use our drink driving sentence calculator to estimate your likely penalty based on your reading.

Driving ban: A minimum 12-month disqualification, rising to 3 years for a second offence within 10 years. Find out how long a disqualification stays on your licence.

Criminal record: A DR10 conviction stays on your licence for 11 years and must be declared to insurers for 5 years.

DVLA medical: If your reading was 2.5 times the limit or higher, you’ll be classified as a High Risk Offender and must pass a DVLA medical including a CDT blood test before getting your licence back.

Insurance: Your premiums will increase substantially. Standard insurers may refuse you, so you’ll need specialist convicted driver insurance. See our guide on how much car insurance costs after a drink driving ban.

If your offence involved drugs rather than alcohol, penalties and insurance implications are different. See our guide to drug driving insurance (DG10, DR80, DR90).