DecayCalculator.com

Nuclear Medicine & Medical Physics Calculators

📐 ICRP 103 compliant 🔬 IAEA half-life data 🔒 Runs locally
☢ Decay Calculator 💉 Activity Concentration Calculator ⚛️ Dose Converter

☢ Radioactive Decay Calculator

📊 Results
🔬 Enter parameters and click
Calculate Activity
📈 Time–Activity Curve
Calculate to generate chart

💉 Activity Concentration Calculator

💉 Enter vial parameters and click
Calculate Volume

⚛️ Dose Unit Converter & Biological Dose Calculator

⚛️Enter a value to convert
Tissue Weighting Factors — ICRP Publication 103 (2007)
Tissue / OrganwTCategory

* Remainder tissues (wT = 0.12 combined): adrenals, extrathoracic region, gall bladder, heart wall, kidneys, lymphatic nodes, muscle, oral mucosa, pancreas, prostate (or uterus/cervix), small intestine, spleen, thymus.   ΣwT = 1.00 ✓

Radiation Weighting Factors — ICRP Publication 103 (2007)
Radiation TypewR

† For neutrons, ICRP 103 defines wR as a continuous function of energy (approximately 2.5–20, peaking near 1 MeV). The value of 10 is used here as an approximation.

Free Radioactive Decay & Dose Calculators for Nuclear Medicine

DecayCalculator.com is a free suite of browser-based tools for medical physics, nuclear medicine, radiology and radiopharmacy. Calculate the remaining activity of a radioisotope after decay, work out the volume of radiopharmaceutical to extract from a vial, and convert between radiation dose units — all in your browser, with no data sent anywhere.

The radioactive decay calculator supports Tc-99m, F-18, I-131, I-125, Ga-67, In-111, Lu-177, Ra-223, Y-90, Ir-192, Co-57, Cs-137, Co-60 and Mo-99, using half-life data from the IAEA and NNDC nuclear data libraries. The activity concentration calculator corrects vial concentration for decay and returns the volume to draw, and the dose unit converter handles Gy, rad, Sv and rem plus ICRP 103 equivalent- and effective-dose calculations.

📋 Isotope Reference Data & Half-Lives

Isotope Full Name Half-Life Decay Constant λ (h⁻¹) Application

❓ Frequently Asked Questions

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. In nuclear medicine, radiopharmaceuticals — drugs labelled with radioactive isotopes — are injected into patients for diagnostic scans or therapeutic treatments. Because these isotopes decay over time, their activity (the rate of radioactive emission) decreases continuously. Technicians and physicists must account for this decay when preparing doses: the activity in a vial at the time of calibration will be different from the activity at the time of administration, sometimes by a significant margin depending on the isotope and elapsed time.
The standard formula is A(t) = A₀ × e−λt, where A₀ is the initial activity, λ is the decay constant (λ = ln(2) / T½), and t is the elapsed time. Our Decay Calculator handles this automatically — just select the isotope, enter the reference activity and calibration time, and set the target time. The calculator also shows a time-activity curve so you can visualise how the activity evolves over a ±48 hour window.
Technetium-99m (Tc-99m) has a half-life of approximately 6.01 hours, which makes it ideal for nuclear medicine. It is short enough that the patient's radiation exposure is limited, but long enough to allow time for radiopharmaceutical preparation, quality control, and the imaging procedure itself. It also emits gamma rays at an energy (140 keV) that is well-suited to gamma camera detection. Tc-99m is the most widely used diagnostic radioisotope in the world, used in bone scans, cardiac perfusion imaging, renal studies, and many other applications.
These are three different ways of quantifying radiation exposure, each more clinically meaningful than the last. Absorbed dose (measured in Gray, Gy) is the raw physical energy deposited per unit mass of tissue. Equivalent dose (measured in Sievert, Sv) adjusts for the biological effectiveness of different radiation types — for example, alpha particles cause more damage per unit of absorbed dose than X-rays. Effective dose (also in Sv) goes one step further and accounts for the varying radiosensitivity of different organs and tissues, allowing a single number to represent the overall health risk from a non-uniform exposure. Our Dose Converter handles all three conversions using ICRP 103 weighting factors.
The tissue weighting factors (wT) defined in ICRP Publication 103 (2007) represent the relative contribution of each organ or tissue to the total radiation detriment from uniform whole-body exposure. Tissues with higher wT values — such as bone marrow, lung, stomach, and colon (each wT = 0.12) — are more radiosensitive and contribute more to the overall effective dose. The sum of all wT values equals 1.00. These factors are used to calculate effective dose: E = Σ(wT × HT), where HT is the equivalent dose to each tissue.
Use our Activity Concentration Calculator. You need to know the concentration of activity in the vial at a reference time (e.g. at the time of elution or delivery), the time at which you will perform the extraction, and the activity you want to administer. The calculator first corrects the concentration for radioactive decay between the reference time and the extraction time, then divides the desired activity by this corrected concentration to give the volume in mL. This is a calculation performed routinely in nuclear medicine departments, particularly for Tc-99m preparations.
These calculators are designed to support and verify manual calculations performed by trained professionals. They use accurate half-life data from the IAEA and NNDC and follow established formulas and international standards. However, they are intended for educational and verification purposes — they do not replace your institution's validated procedures, local regulations, or professional judgment. Always confirm doses and volumes using your institution's approved methods before administration.
1 millicurie (mCi) equals exactly 37 megabecquerels (MBq), because 1 curie = 3.7 × 1010 becquerels by definition. To convert mCi to MBq, multiply by 37 (e.g. 10 mCi = 370 MBq). To convert MBq to mCi, divide by 37. Other useful equivalences: 1 µCi = 0.037 MBq = 37 kBq, and 1 GBq ≈ 27.027 mCi. You can switch between MBq, GBq, mCi, µCi, kBq and Bq directly inside the Radioactive Decay Calculator.
Fluorine-18 (F-18) has a half-life of approximately 109.77 minutes (1.83 hours). It is the most common positron emitter in PET imaging and is used to label FDG (fluorodeoxyglucose, 18F-FDG) for oncology, cardiology and neurology scans. Because of its short half-life, FDG activity must be decay-corrected carefully between calibration, delivery and injection — the Radioactive Decay Calculator handles this for F-18.
Lutetium-177 (Lu-177) has a half-life of approximately 6.647 days (159.46 hours). It is a beta- and gamma-emitting therapeutic radionuclide used in targeted radionuclide therapy, including Lu-177 DOTATATE for neuroendocrine tumours and Lu-177 PSMA-617 for metastatic prostate cancer. Select Lu-177 in the Radioactive Decay Calculator to decay-correct therapy activities.
Iodine-131 (I-131) has a half-life of approximately 8.02 days (192.5 hours). It is used both for therapy (thyroid cancer and hyperthyroidism) and for imaging, emitting beta particles and a 364 keV gamma ray. Use the Radioactive Decay Calculator to find the remaining I-131 activity between calibration and administration.
1 gray (Gy) = 100 rad, so to convert Gy to rad multiply by 100 (and divide by 100 for rad to Gy). The same factor applies to dose equivalent: 1 sievert (Sv) = 100 rem. So 0.05 Gy = 5 rad and 2 mSv = 0.2 rem. The Dose Unit Converter converts between Gy, mGy, µGy, rad, mrad, Sv, mSv, µSv, rem and mrem instantly.
The decay constant λ is calculated from the half-life using λ = ln(2) / T½ = 0.6931 / T½. It represents the probability of decay per unit time. For example, Tc-99m with a 6.0067-hour half-life has λ = 0.6931 / 6.0067 ≈ 0.1154 h−1. The Isotope Reference Data table lists the decay constant for every supported isotope.
Decay correction adjusts a measured or reference activity to a different point in time to account for radioactive decay. The decay-correction factor is e−λt, where t is the time elapsed (positive for a later time, negative for an earlier time). Multiply the reference activity by this factor to obtain the corrected activity. The Radioactive Decay Calculator performs this correction automatically for any reference and target time.

📖 Quick Reference — Key Formulas

Radioactive Decay

A(t) = A₀ × e−λt

A₀ = initial activity · t = elapsed time · λ = decay constant

Decay Constant

λ = ln(2) / T½

T½ = half-life of the radioisotope

Activity Concentration (decayed)

C(t) = C₀ × e−λt

C₀ = reference concentration · t = elapsed time

Volume to Extract

V = Adesired / C(t)

Adesired = required activity · C(t) = concentration at extraction time

Equivalent Dose

H = wR × D

D = absorbed dose (Gy) · wR = radiation weighting factor (ICRP 103)

Effective Dose

E = wT × H = wT × wR × D

H = equivalent dose (Sv) · wT = tissue weighting factor (ICRP 103)

About these tools

DecayCalculator.com provides free, browser-based calculators designed for professionals working with radioactive materials in clinical and research settings. Whether you need to know the remaining activity of a Tc-99m vial before a scan, calculate the volume to extract from a radiopharmaceutical preparation, or convert between radiation dose units for a patient report, these tools are built for your daily workflow.

All calculators use accurate half-life data sourced from the IAEA and NNDC nuclear data libraries, and biological dose calculations follow ICRP Publication 103 (2007) — the current international standard for radiation protection.