This is for a test that already happened. If you are trying to work out whether you are safe to drive right now, close this page. No calculator can tell you that, the honest answer is always no if you are asking, and I am not going to pretend otherwise.
What this is for is the morning after, when the machine printed a number that does not match the night you remember. Four or five drinks over an evening should not produce a 0.19. When the arithmetic and the instrument disagree, that gap is worth understanding, because it points at the records that decide breath cases.
Widmark uses different constants. It is a crude split and one of the model’s known weaknesses.
Your weight around the time of the arrest.
One standard drink is 12 oz of regular beer, 5 oz of wine, or 1.5 oz of spirits. A double, a strong IPA or a generous home pour counts as more than one.
From the first drink of the night to when you actually blew, not to when you were stopped.
Optional, but it unlocks the better model. 5 ft 10 in is 70.
Optional. Needed for the Watson calculation.
Optional. What the machine printed.
Why the answer is a range, and why two models
Widmark is the century-old equation, and it is what a State toxicologist reaches for. It turns on a single ratio standing in for how much of your body is water: conventionally 0.68 for men and 0.55 for women, with a spread of about plus or minus 0.085 and 0.055 respectively. This tool runs it across that whole spread rather than at the midpoint, and applies it symmetrically, so the band is not quietly tilted in your favour.
Watson is the better model, and the one worth holding an expert to. Rather than assuming a ratio, it estimates your actual total body water from height, weight, age and sex, then puts the alcohol into that volume. It predicts the resulting level a good deal more accurately than a single borrowed ratio. Give the tool your height and age and it fills in beside Widmark, and where the two disagree tells you something on its own.
Elimination is a band too, roughly 0.012 to 0.020 per hour rather than the tidy 0.015 you see quoted. That is not fussiness. A fixed clearance rate is precisely the assumption the State needs to run a reading backwards to the time of driving, and it is not a safe thing to hand over.
Most calculators pick the middle of every spread and print one confident number. That is the wrong thing to do with models like these, because it implies a precision neither one has.
It also means the tool cuts both ways, and it should. If your drink count genuinely produces a number near what the machine reported, you should know that, and no arithmetic will change it.
My own treatise is hard on this model
It is, and it would be dishonest to run a calculator on it without saying so.
To be clear about which book: this is The Florida DUI Trial Manual, the one I wrote for other lawyers rather than for clients. It is a working reference for taking the State’s science apart, and its first chapter spends a good deal of time on exactly this equation. (The plain-English book for people facing a charge is Arrested for DUI in Florida, which is free to Tampa Bay residents with a pending case.)
The Widmark ratio was built almost a century ago from a small group of subjects. It gets printed to three decimal places, but the precision is for show. It is an average dressed up as a measurement, and nothing about it is guaranteed to match you. That is the whole reason Watson sits next to it here rather than in a footnote.
The caution runs against the defense as well. I do not argue that the straight line fails outright, because inside the range most cases live in it is a decent approximation, and that is the one place a well prepared State expert can open his own book and win. What I will not concede is a fixed elimination rate, because that is the premise the whole backward calculation needs.
So why publish it? Because the field’s own consensus standard for these calculations does not ask for a number, it asks for a range with the assumptions travelling alongside it. The State’s own forensic reference, reviewing the challenges to breath testing, declines to defend back-calculation and calls it fraught with significant uncertainty. This tool is built to those criticisms rather than in spite of them.
One distinction worth keeping straight. This runs forward, from what you say you drank to a predicted level. The State runs backward, from a reading to a claimed level hours earlier. The backward version carries every weakness here and adds more, starting with the fact that without your last drink time and your last meal time nobody can place you on the falling side of the curve at all. Neither is usually in a routine DUI file.
What it assumes, and where that breaks
That everything was absorbed. The model treats all the alcohol as already in your blood. If you were still drinking close to when you drove, your real curve was still climbing, and the level at the wheel was lower than the level at the machine. That gap is a defense in itself, and it is covered on the rising BAC page.
That you counted honestly and accurately. Drink strength is the single largest source of error here. Almost nobody pours a standard drink at home, and a strong beer can be nearly two. If your count is low, the estimate is low.
That the machine measured what it claims. Every breath result rests on an assumed ratio between alcohol in the blood and alcohol in the breath, applied to you as though it had been measured on you. It has not been. That assumption is explained on the partition ratio page.
This is for alcohol. Drugs cannot be run backward at all
Everything above is ethanol arithmetic. None of it transfers to a drug case, and the reason is not a defense talking point. It is the forensic toxicology field’s own standard.
The guideline on opinions and testimony in forensic toxicology, ANSI/ASB Best Practice Recommendation 037, directs that a toxicologist should not offer a back-extrapolation opinion for anything other than ethanol. The absorption and elimination of drugs are too variable and too poorly mapped to run backward at all. So there is no legitimate way to take a drug concentration measured hours after driving and testify to what it was at the wheel.
That matters because it is the standard the State’s own expert is held to. A competent State toxicologist will concede it, because conceding it is what her own guidelines require. The question is whether anyone asks.
The same guideline reaches a second move: you cannot read a degree of impairment off a drug level either. A number is not a behaviour, and for most substances there is no level that corresponds to impairment the way 0.08 does for alcohol. That is the whole ground of presence versus impairment.
And the specimen matters. A urine result is not a blood result and does not become one by assertion. Urine cannot be converted to whole blood, and it cannot be run backward. Where a case rests on a urine sample, or on a serum draw pushed through a conversion factor, each of those steps folds its own error into the single tidy figure printed on the report.
What actually wins these
And where the timing genuinely is the case, you hire an expert. If the fight is really about what your level was at the wheel rather than at the machine, a website calculator is not the answer and neither is a lawyer doing arithmetic on a legal pad. The answer is a qualified forensic toxicologist who can run the calculation properly on the actual record, your last drink time, your last meal, the instrument data, the real interval, and then say so under oath and be cross-examined on it.
I retain one where a case turns on it. That is a genuine expense and I will tell you plainly whether yours justifies it, because most cases do not and it is not a cost worth adding for its own sake. But when the State puts its own toxicologist up to run a reading backwards to the time of driving, that opinion has to clear the reliability standard for expert testimony, and the method and its application to your facts are both fair game. Meeting a State expert with nothing is a choice, and it is usually the wrong one.
Ask any lawyer you interview whether an expert is included in the fee or billed separately. It is one of the questions worth asking, and the answer tells you how the case will actually be run.
Not this. A defense lawyer does not walk into court with a calculator. What moves a breath case is the paper the instrument generates: the agency inspections, the department inspections, the operator’s permit, and the machine’s own error and interferent logs, all tied to the serial number of the specific instrument that tested you. Plus the twenty minute observation that either happened properly or did not.
Where this tool earns its place is in telling you whether there is something to look for. A reading well above what your night should have produced is a reason to pull all of it. The case evaluator walks through the rest of the case the same way, and the breath test pages explain what each record is for.
This is general information about a scientific model, not legal advice, and it is not evidence of anything. It cannot tell you what your level was, only what a population model would predict from the numbers you entered. If the machine’s number and your night do not line up, call or text me at (727) 761-4318 and we can talk about what the records show. Every case is different, and past results do not guarantee a similar outcome.
