Drug half-life calculator

Six half-lives
Half-lives	Elapsed hours	Amount remaining (mg)
0.000	0.000	100.000
1.000	6.000	50.000
2.000	12.000	25.000
3.000	18.000	12.500
4.000	24.000	6.250
5.000	30.000	3.125
6.000	36.000	1.563

Six half-lives

Half-lives

Elapsed hours

Amount remaining (mg)

0.000

100.000

1.000

6.000

50.000

2.000

12.000

25.000

3.000

18.000

12.500

4.000

24.000

6.250

5.000

30.000

3.125

6.000

36.000

1.563

First-order elimination curve data
Series	Half-lives elapsed	Amount remaining (mg)
Amount remaining	0	100
Amount remaining	0.1	93.30329915368074
Amount remaining	0.2	87.05505632961241
Amount remaining	0.3	81.22523963562355
Amount remaining	0.4	75.7858283255199
Amount remaining	0.5	70.71067811865474
Amount remaining	0.6	65.9753955386447
Amount remaining	0.7	61.557220667245815
Amount remaining	0.8	57.43491774985174
Amount remaining	0.9	53.588673126814655
Amount remaining	1	50
Amount remaining	1.1	46.65164957684037
Amount remaining	1.2	43.527528164806206
Amount remaining	1.3	40.612619817811776
Amount remaining	1.4	37.89291416275996
Amount remaining	1.5	35.35533905932737
Amount remaining	1.6	32.98769776932235
Amount remaining	1.7	30.778610333622908
Amount remaining	1.8	28.71745887492587
Amount remaining	1.9	26.794336563407327
Amount remaining	2	25
Amount remaining	2.1	23.325824788420185
Amount remaining	2.2	21.7637640824031
Amount remaining	2.3	20.306309908905888
Amount remaining	2.4	18.94645708137998
Amount remaining	2.5	17.677669529663685
Amount remaining	2.6	16.493848884661176
Amount remaining	2.7	15.38930516681145
Amount remaining	2.8	14.358729437462939
Amount remaining	2.9	13.397168281703664
Amount remaining	3	12.5
Amount remaining	3.1	11.662912394210093
Amount remaining	3.2	10.88188204120155
Amount remaining	3.3	10.153154954452944
Amount remaining	3.4	9.47322854068999
Amount remaining	3.5	8.838834764831843
Amount remaining	3.6	8.246924442330588
Amount remaining	3.7	7.694652583405725
Amount remaining	3.8	7.1793647187314695
Amount remaining	3.9	6.698584140851832
Amount remaining	4	6.25
Amount remaining	4.1	5.831456197105048
Amount remaining	4.2	5.440941020600775
Amount remaining	4.3	5.076577477226472
Amount remaining	4.4	4.736614270344993
Amount remaining	4.5	4.419417382415921
Amount remaining	4.6	4.123462221165296
Amount remaining	4.7	3.8473262917028626
Amount remaining	4.8	3.5896823593657348
Amount remaining	4.9	3.3492920704259155
Amount remaining	5	3.125
Amount remaining	5.1	2.915728098552524
Amount remaining	5.2	2.7204705103003874
Amount remaining	5.3	2.538288738613236
Amount remaining	5.4	2.3683071351724965
Amount remaining	5.5	2.2097086912079607
Amount remaining	5.6	2.061731110582648
Amount remaining	5.7	1.9236631458514313
Amount remaining	5.8	1.7948411796828674
Amount remaining	5.9	1.6746460352129577
Amount remaining	6	1.5625

First-order elimination curve data

Series

Half-lives elapsed

Amount remaining (mg)

Amount remaining

100

Amount remaining

0.1

93.30329915368074

Amount remaining

0.2

87.05505632961241

Amount remaining

0.3

81.22523963562355

Amount remaining

0.4

75.7858283255199

Amount remaining

0.5

70.71067811865474

Amount remaining

0.6

65.9753955386447

Amount remaining

0.7

61.557220667245815

Amount remaining

0.8

57.43491774985174

Amount remaining

0.9

53.588673126814655

Amount remaining

1.1

46.65164957684037

Amount remaining

1.2

43.527528164806206

Amount remaining

1.3

40.612619817811776

Amount remaining

1.4

37.89291416275996

Amount remaining

1.5

35.35533905932737

Amount remaining

1.6

32.98769776932235

Amount remaining

1.7

30.778610333622908

Amount remaining

1.8

28.71745887492587

Amount remaining

1.9

26.794336563407327

Amount remaining

2.1

23.325824788420185

Amount remaining

2.2

21.7637640824031

Amount remaining

2.3

20.306309908905888

Amount remaining

2.4

18.94645708137998

Amount remaining

2.5

17.677669529663685

Amount remaining

2.6

16.493848884661176

Amount remaining

2.7

15.38930516681145

Amount remaining

2.8

14.358729437462939

Amount remaining

2.9

13.397168281703664

Amount remaining

12.5

Amount remaining

3.1

11.662912394210093

Amount remaining

3.2

10.88188204120155

Amount remaining

3.3

10.153154954452944

Amount remaining

3.4

9.47322854068999

Amount remaining

3.5

8.838834764831843

Amount remaining

3.6

8.246924442330588

Amount remaining

3.7

7.694652583405725

Amount remaining

3.8

7.1793647187314695

Amount remaining

3.9

6.698584140851832

Amount remaining

6.25

Amount remaining

4.1

5.831456197105048

Amount remaining

4.2

5.440941020600775

Amount remaining

4.3

5.076577477226472

Amount remaining

4.4

4.736614270344993

Amount remaining

4.5

4.419417382415921

Amount remaining

4.6

4.123462221165296

Amount remaining

4.7

3.8473262917028626

Amount remaining

4.8

3.5896823593657348

Amount remaining

4.9

3.3492920704259155

Amount remaining

3.125

Amount remaining

5.1

2.915728098552524

Amount remaining

5.2

2.7204705103003874

Amount remaining

5.3

2.538288738613236

Amount remaining

5.4

2.3683071351724965

Amount remaining

5.5

2.2097086912079607

Amount remaining

5.6

2.061731110582648

Amount remaining

5.7

1.9236631458514313

Amount remaining

5.8

1.7948411796828674

Amount remaining

5.9

1.6746460352129577

Amount remaining

1.5625

How this calculator works

This model removes the same fraction in each half-life, with a constant elimination rate and no further dosing. The initial model amount means amount in the body after absorption, not necessarily the swallowed dose. Distribution, active metabolites, nonlinear kinetics and changes in clearance are omitted. A small remaining fraction does not imply no clinical effect. The result is an estimate for information only and is not medical advice.

Formula

A(t) = A0 x 2^(-t/half-life). Fraction = A/A0. t = -half-life x log2(fraction); half-life = -t/log2(fraction).

Worked example

With an initial amount of 100 mg and a 6-hour half-life, 25 mg remains after 12 hours. After 30 hours, five half-lives have passed and 3.125 mg remains.

Frequently asked questions

Is the amount completely gone after five half-lives?

No. This ideal model leaves 3.125% after five half-lives and never reaches exactly zero at finite time.

Does this model repeated doses?

No. It models elimination from one initial amount without further input or absorption.

Can it decide a washout period?

No. Required washout depends on the actual substance, active metabolites, effects and individual physiology.

Sources

Written byAmira MansourCalculator writer. Builds and tests Mathulate calculators against published sources.

Reviewed byPriya Natarajan, RDRegistered dietitian. Checks health calculators against public health guidance.

Last updated November 1, 2026.

Drug half-life calculator

Calculator

How this calculator works

Formula

Worked example

Frequently asked questions

Sources

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