Drug Half Life Calculator

Use the Drug Half Life Calculator

Estimate elimination time, fraction remaining, and steady‑state context from a known half‑life. The drug half life calculator helps plan safe dosing intervals.

Start your half‑life estimate

How to Use Drug Half Life Calculator

1. Step 1: Enter half‑life

   Type the drug's half‑life and select minutes, hours, or days.

2. Step 2: Add timing

   Enter time since your last dose to see fraction remaining.

3. Step 3: Optional dose & interval

   Add dose (mg) to estimate mg remaining and dosing interval to see accumulation.

4. Step 4: Set target elimination

   Choose a target like 95% to compute a specific washout time.

5. Step 5: Calculate & review

   Press Calculate and review the summary cards and guidance.

Key Features

• Half-life calculation
• Elimination time
• Steady state guidance
• Dosing interval help

Understanding Results

Formula

This tool uses a first‑order elimination model. If a medication's half‑life is t_½, the elimination constant is k = ln(2) / t_½. The fraction remaining after time t is e^−k·t, which is the same as (1/2)^{t / t_½}. To find time to a target level, rearrange to t = t_½ × log₂(1/p), where p is the remaining fraction (e.g., 0.05 for 95% eliminated).

Reference Ranges & Interpretation

Practical landmarks: ~3.32 half‑lives for 90% eliminated, ~4.32 for 95%, ~5.32 for 97.5%, and ~6.64 for 99%. These points help you estimate washout and the time to near steady state when dosing regularly. Effects may persist longer than the molecule if downstream pathways take time to reset.

Assumptions & Limitations

Estimates assume first‑order kinetics without prolonged absorption. Individual factors (age, renal/hepatic function, genetics, formulation, and interactions) can lengthen or shorten effective half‑life. Numbers are educational and not a substitute for clinical judgment or prescribing information.

Complete Guide: Drug Half Life Calculator

Use the drug half life calculator to estimate medication elimination time and plan safe dosing intervals based on half-life and accumulation. Accurate results.

In clinical pharmacology, half‑life is a practical way to think about how fast a medication leaves the body. After one half‑life, about half the drug remains; after two, about one‑quarter remains; after five, only a few percent are left. Our drug half life calculator turns those ideas into concrete times, so you can estimate washout, plan dosing intervals, and understand when a steady level is reached with regular dosing. The explanations below keep the math transparent and the language simple.

What half‑life means in plain English

A drug’s elimination half‑life is the amount of time it takes for the concentration in the body to drop to half of its current level. If a medicine has a half‑life of 8 hours, then 8 hours after a dose, about 50% remains; 16 hours after a dose, roughly 25% remains; 24 hours later, about 12.5% remains, and so on. This pattern is exponential: the amount decreases by half over each equal block of time. Half‑life is not a guarantee for one person but an average measured in studies. It helps set expectations for when a dose wears off, how often a medicine might be taken, and how long before most of it is gone.

Because half‑life is time‑based, you can convert it to percent eliminated. Five half‑lives correspond to about 97% eliminated; 6–7 half‑lives approach 99–99.2%. That’s why many sources say “steady state” (a stable pattern of highs and lows with repeated dosing) is reached in about 4–5 half‑lives. These are rules of thumb, not medical advice.

How the drug half‑life calculator works

Enter the drug’s half‑life and choose units. Optionally, enter the time since your last dose and the dose size (mg). The tool estimates the fraction remaining at that time and, if a dose is entered, the milligrams likely still present. You can also add a dosing interval (for example every 12 or 24 hours). With that interval, the calculator shows the accumulation ratio — an estimate of how much higher the steady‑state level is compared with the level after a single dose. Finally, set a target percentage eliminated (like 95%). The tool computes how long it typically takes to reach that level of washout.

All calculations assume a simple one‑compartment, first‑order elimination model (the most common simplifying model in non‑emergency contexts). That means the rate of elimination is proportional to the current amount, so the decline follows a smooth curve. It does not account for situations where absorption is prolonged, where distribution into tissues is very slow, or where metabolism saturates.

The formula and equations behind the tool

The heart of the calculator is the relationship between half‑life and the elimination rate constant. If t_½ is the half‑life, then the first‑order elimination constant is k = ln(2) / t_½. The fraction remaining at a time t after a dose is e^−k·t, which is equivalent to (1/2)^{t / t_½}. To get time for a target fraction remaining p, rearrange the equation: t = t_½ × log₂(1/p). If you are interested in percent eliminated instead, use p = 1 − (percent eliminated/100).

Common waypoints are easy to remember: about 3.32 half‑lives for 90% eliminated, 4.32 for 95%, 5.32 for 97.5%, and 6.64 for 99%. The tool displays those times automatically in hours and in your chosen unit so it’s easy to compare with your schedule.

Time to 90%–99% eliminated

Many everyday questions boil down to “When is most of the drug out of my system?” While “most” is subjective, 90% to 99% are conventional reference points. They show when only a small fraction remains and when accumulation from prior doses is negligible for many use cases. You can select any target between these values. In practice, 95% offers a good balance: it usually aligns with the “about five half‑lives” rule of thumb and is commonly used in clinical summaries. Remember that a tiny fraction can still be present even after long periods, and effects can last longer than the molecule if downstream pathways or receptors take time to reset.

If you enter a last dose amount (mg), the calculator estimates how many milligrams likely remain. This number is only meaningful for context — it is not a dose recommendation. It is especially helpful when planning a taper or spacing doses. If you are exploring dose planning, you may also find our dosage calculator and mg to mL dose calculator useful companions.

Repeated dosing and accumulation (R)

When you take repeated doses, each dose adds to what was left from the prior dose. With first‑order elimination, the average size of that build‑up is captured by the accumulation ratio, often written as R = 1 / (1 − e^−k·τ), where τ (tau) is the dosing interval. If the interval is long compared with the half‑life, the exponential term is almost zero and R is close to 1 (little accumulation). If the interval is short compared with the half‑life, R climbs, meaning higher levels at steady state than after the first dose. Our calculator shows this number so you can see the direction and magnitude — again, for context, not for making decisions in place of a clinician.

If two different intervals fit your daily routine, compare the accumulation ratios. A longer interval usually reduces accumulation. However, the right interval depends on factors outside the scope of this tool: symptom control, peak‑trough effects, and clinical guidance. For pain‑management topics, you might also explore our opioid conversion calculator and morphine equivalent calculator for safe comparison between agents. Always follow prescribing instructions.

Reaching steady state and washout

With regular dosing at a fixed interval, levels typically approach a repeating pattern known as steady state. For first‑order kinetics, the time to near steady state is governed by half‑life — not by the dose size or the interval. A useful landmark is about 4–5 half‑lives to get close (roughly 94–97% of the way there). The same rule works in reverse for washout after you stop taking a medicine. The shape of the curve is mirrored; it just runs backward.

This simple rule helps you plan restarts and tapers with your care team. For example, if a medication has a half‑life of 36 hours, expect around a week to approach steady state and a similar timeframe for a near‑complete washout. If kidney or liver function is reduced, half‑life can be longer. You can estimate renal function with our creatinine clearance calculator or eGFR calculator and discuss with a clinician whether adjustments are appropriate.

Practical examples and planning tips

Suppose a medication’s half‑life is 8 hours and you take 50 mg. The calculator shows that after 24 hours (3 half‑lives), about 12.5 mg (25%) remain. If you dose every 24 hours, the accumulation ratio is near 1.33; if you dose every 8 hours, it rises, because the interval equals the half‑life. That doesn’t mean short intervals are “bad” — sometimes they smooth out symptoms — but it does explain why levels at steady state can be higher than after the very first dose.

If you’re switching dosing schedules, you can use the tool to compare the time to 95% eliminated between the last dose on the old schedule and the first dose on the new one, leaving an appropriate gap when directed by a clinician. For liquid medicines, our mg ↔ mL calculator can reduce arithmetic mistakes when measuring out a new volume. For pediatric contexts, visit the pediatric dose calculator, and for older adults, the geriatric dose calculator for age‑aware dosing considerations.

Another example: If your clinician asks you to hold a medicine for a test or procedure, a common question is “How long should I pause?” The calculator’s time‑to‑elimination table gives you a user‑set target like 95% or 97.5%. Combine that with the medication’s known half‑life (from reputable sources) to have a clear conversation about lead times. This is not medical advice, but a planning aid to frame questions precisely.

When results vary from expectations

Half‑life numbers come from populations, not from you specifically. Genetics, age, body composition, liver and kidney function, smoking status, drug interactions, and even time of day can move the effective half‑life up or down. Diseases that slow gastric emptying can delay absorption for oral medications, making the early decline look different from the simple model. Depot injections, extended‑release tablets, and transdermal patches introduce a long absorption phase that can mask elimination. In these cases, “time since last dose” might not equal “time since true peak in the bloodstream.”

Drug–drug interactions are a common source of surprises. Enzyme inducers and inhibitors can shorten or extend half‑life; protein binding displacement can alter the fraction that is pharmacologically active. Always check for interactions using official resources or with your pharmacist. If you see a discrepancy between expected and actual effects, clinical judgment is paramount. The tool is not designed for emergencies or for drugs with narrow therapeutic windows.

Useful related calculators

These tools complement the drug half life calculator and can help you answer adjacent questions with the same clean, mobile‑first experience:

• Dosage Calculator — general arithmetic for once‑off dose planning.
• Dose Calculator (mg ↔ mL) — convert milligrams to milliliters and back.
• Pediatric Dose Calculator — weight‑aware guidance for children.
• Geriatric Dose Calculator — age‑aware checking for older adults.
• Creatinine Clearance Calculator — estimate renal function (Cockcroft‑Gault).
• eGFR Calculator — estimated glomerular filtration rate.
• Opioid Conversion Calculator — compare between opioid agents when directed.

Sources, assumptions, and limitations

This guide is educational and does not give medical advice. It leverages a standard, first‑order elimination model. Equations are drawn from basic clinical pharmacology texts and common references. For consumer‑facing drug information, review FDA medication guides and authoritative monographs. For advanced reading, the NIH’s NCBI Bookshelf is a strong starting point.

External references (non‑commercial): FDA Drugs and NCBI Bookshelf.

Numbers from the calculator should be treated as estimates that help you reason clearly and ask precise questions. They are not dosing instructions. For medication changes, timing around procedures, and safety‑critical decisions, consult a qualified clinician or pharmacist.

Frequently Asked Questions