How to read the calculator
Choose a radionuclide or enter a custom physical half-life. Enter the known activity, elapsed time and direction. Results show the calculated activity in two units and the decay factor.
The decay law
A = A₀ × 2^(−t/T½), where t is positive after the reference time and negative before it.
Rearranging the same equation gives t = T½ × log₂(A₀/A), for positive activities expressed in the same unit. A positive t is after the reference time; a negative t is before it.
Decay-table example: TECHNELITE, Table 4 (Physical Decay Chart: Technetium Tc 99m Half-Life 6 Hours), 6 hours: 50 percent remaining. The starting activity is hypothetical.
| Item | Value |
|---|---|
| Radionuclide | Tc-99m |
| Half-life | T½ 6.0067 h (DDEP; table dated 2012). |
| Activity at the reference time | 1000 MBq |
| Time from the reference | 6 h |
| Direction | After |
| Activity 6 h after the reference time | 500.4 MBq (13.52 mCi) |
| Decay factor | 0.5004 (6 h · 0.9989 half-lives) |
Starting activities are hypothetical round numbers chosen to illustrate the verified decay fraction. They are not administered-activity examples.
Physical decay only, assuming no material is added or removed; excludes biological clearance and daughter ingrowth.
Half-lives used
Preset half-lives use DDEP recommended data checked against the cited tables on 5 November 2026. Each preset identifies the year of its table’s later footer date.
DDEP notes that the Tc-99m half-life can vary slightly with chemical environment; this calculator uses the single recommended value.
Activities and decay factors are rounded to 4 significant figures. Displayed digits do not express measurement uncertainty; half-life uncertainties are not propagated.
| Radionuclide | DDEP half-lifeDecay Data Evaluation Project: evaluated radionuclide decay data published by LNE-LNHB and partners. | Table date | Source |
|---|---|---|---|
| F-18 | 1.82890 h | 2014 | LNE-LNHB F-18 table |
| Ga-68 | 67.83 min | 2012 | LNE-LNHB Ga-68 table |
| Tc-99m | 6.0067 h | 2012 | LNE-LNHB Tc-99m table |
| Mo-99 | 2.7479 d | 2012 | LNE-LNHB Mo-99 table |
| I-123 | 13.2234 h | 2004 | LNE-LNHB I-123 table |
| In-111 | 2.8049 d | 2006 | LNE-LNHB In-111 table |
| Ga-67 | 3.2613 d | 2011 | LNE-LNHB Ga-67 table |
| Tl-201 | 3.0421 d | 2005 | LNE-LNHB Tl-201 table |
| Xe-133 | 5.2474 d | 2008 | LNE-LNHB Xe-133 table |
| I-131 | 8.0233 d | 2014 | LNE-LNHB I-131 table |
| Lu-177 | 6.6443 d | 2025 | LNE-LNHB Lu-177 table |
| Y-90 | 2.6684 d | 2015 | LNE-LNHB Y-90 table |
| Ra-223 | 11.43 d | 2011 | LNE-LNHB Ra-223 table |
| Cu-64 | 12.7004 h | 2011 | LNE-LNHB Cu-64 table |
Why a package insert can differ
Package inserts may state a different half-life or print a coarsely rounded decay table. This calculator uses the displayed DDEP preset value. For example, TECHNELITE states 6.02 h for Tc-99m, compared with the DDEP value of 6.0067 h; using those two values gives about a 0.77% difference after five DDEP half-lives.
NETSPOT states 68 min in its text. Its printed decay table is consistent, at the table’s precision, with 67.71 min.
XOFIGO states 11.4 d in its text. Its decay table is consistent with 11.43 d and includes the label’s fixed 7-hour CET-to-CST offset.
Before the reference time
For a positive elapsed time, Before gives a factor above 1 and After gives a factor below 1. At zero elapsed time, the factor is 1.
Decay-table example: OCTREOSCAN, Table 3 (Physical Decay Chart: Indium In-111, Half-life 2.805 Days (67.32 hours)), −24 hours: fraction remaining 1.280. The starting activity is hypothetical.
| Item | Value |
|---|---|
| Radionuclide | In-111 |
| Half-life | T½ 2.8049 d (DDEP; table dated 2006). |
| Activity at the reference time | 100 MBq |
| Time from the reference | 24 h |
| Direction | Before |
| Activity 24 h before the reference time | 128.0 MBq (3.460 mCi) |
| Decay factor | 1.280 (1 d · 0.3565 half-lives) |
Fraction remaining after n half-lives
| Half-lives elapsed | Fraction remaining |
|---|---|
| 1 | 0.5 |
| 2 | 0.25 |
| 3 | 0.125 |
| 4 | 0.0625 |
| 5 | 0.03125 |
| 6 | 0.015625 |
| 7 | 0.0078125 |
| 8 | 0.00390625 |
| 9 | 0.001953125 |
| 10 | 0.0009765625 |
Time zones
Enter the actual elapsed time, accounting for time zones and daylight-saving changes. h:mm means elapsed hours and minutes, not a time of day. This calculator does not subtract clock times.
What it does not calculate
This page calculates activity only. It does not calculate radiation dose, biological clearance, daughter ingrowth or generator output.
The 100-half-life and activity limits are interface limits. Inputs outside them produce no result. A positive calculation that cannot be represented reliably also produces no result, rather than zero or infinity.
Frequently asked questions
How do you calculate radioactive decay?
Use A = A₀ × 2^(−t/T½), where A₀ is the activity at the reference time and T½ is the physical half-life. Signed time t is positive after the reference time and negative before it.
What is the half-life of Tc-99m?
The DDEP recommended value used here is 6.0067 h. TECHNELITE states 6.02 h in its text, while its physical decay table is titled ‘Half-Life 6 Hours’.
How do you decay-correct to an earlier time?
Enter the elapsed interval and choose Before. For a positive interval, the calculated factor is above 1. At zero elapsed time, it is 1.
Why does my package insert’s decay chart differ slightly?
The package insert may state a different half-life or round its table values differently. This calculator uses the displayed DDEP preset value, or the custom half-life you enter.
Does the calculator include biological clearance or daughter ingrowth?
No. It calculates physical decay of the selected radionuclide only.
What is a decay factor?
The decay factor is A ÷ A₀ = 2^(−t/T½). Multiply the activity at the reference time by this factor to obtain the calculated activity at the target time.
References
- LNE-LNHB/CEA. Table de Radionucléides: Decay Data Evaluation Project (DDEP) recommended data, nuclide tables for F-18, Ga-68, Tc-99m, Mo-99, I-123, In-111, Ga-67, Tl-201, Xe-133, I-131, Lu-177, Y-90, Ra-223 and Cu-64 (§2 Nuclear Data). https://www.lnhb.fr/home/nuclear-data/nuclear-data-table/. Checked 5 November 2026.
- Bé M-M, Coursol N, Duchemin B, Lagoutine F, Legrand J, Debertin K, Schönfeld E. Table de Radionucléides: Introduction. Note technique LNHB 2011/53. CEA Saclay, LIST, LNE-LNHB, December 2011. §3.1. https://www.lnhb.fr/pdf/DDEP/Introduction_2011.pdf.
- NIST. Radionuclide Half-Life Measurements, NIST Standard Reference Database 120 (updated January 18, 2022). https://www.nist.gov/pml/radiation-physics/radioactivity/radionuclide-half-life-measurements.
- Bureau International des Poids et Mesures. The International System of Units (SI), 9th edition 2019, V4.01 (June 2026). Appendix 1: 12th CGPM (1964) Resolution 7. doi:10.59161/AUEZ1291.
- Thompson A, Taylor BN. Guide for the Use of the International System of Units (SI). NIST Special Publication 811, Appendix B.9. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9.
- 10 CFR § 20.1005 Units of radioactivity. Electronic Code of Federal Regulations, current to 3 November 2026. https://www.ecfr.gov/current/title-10/section-20.1005.
- TECHNELITE (Technetium Tc 99m generator) prescribing information, SHINE SPECT USA. DailyMed SPL setid 626f10b5-cdbf-45b9-9cba-59a3c15f98bf, version 1, effective 2 September 2026. Tables 3 and 4. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=626f10b5-cdbf-45b9-9cba-59a3c15f98bf.
- OCTREOSCAN (kit for the preparation of indium In-111 pentetreotide) prescribing information, Curium. DailyMed SPL setid 93d8f3b2-1216-41dc-a63d-0e812b33891d, version 17, effective 21 February 2022. Table 3. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=93d8f3b2-1216-41dc-a63d-0e812b33891d.
- NETSPOT (kit for the preparation of gallium Ga 68 dotatate injection) prescribing information, Advanced Accelerator Applications. DailyMed SPL setid b2b3be70-17d8-4093-896c-f1c54a2cf242, version 23, effective 6 July 2026. Table 7. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b2b3be70-17d8-4093-896c-f1c54a2cf242.
- XOFIGO (radium Ra 223 dichloride) prescribing information, Bayer. DailyMed SPL setid a398400e-bd31-41a9-9696-4f7c06569ede, version 17, effective 2 March 2026. Table 1. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a398400e-bd31-41a9-9696-4f7c06569ede.