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Black tea before bed: 47 mg of caffeine and how long it lasts

Updated October 5, 2026 · Sources: Mayo Clinic · U.S. Department of Agriculture · U.S. Food and Drug Administration · Foods · National Academies Press

Short answer

47 mg

per 8 fl oz (237 ml)

4 h 33 min

until under 25 mg is left

A cup of black tea starts at 47 mg and needs about 4 h 33 min to drop under 25 mg. That is roughly half the wait a brewed coffee asks for, so a tea habit can run closer to bedtime. The catch is the mug: the size you pour matters more than the brand.

Range in site data25–48 mgby brand, brew and size

Share of the FDA's 400 mg a day12%one serving, healthy adults

General information from population studies (NIH, CDC, NSF, AASM). It is not medical advice and does not replace a clinician who knows you.

Small dose

47 mg is under the 50 mg line this site uses for a small dose, but tea rarely stops at one mug. Two 8 fl oz mugs come to about 94 mg, and the clock then reads about 9 h 30 min to get under 25 mg. Count mugs, not sittings, before you pick a finish-by time.

Caffeine left after one black tea (47 mg)

Model: remaining = dose × 0.5^(hours ÷ 5). Half-life 5 h (Institute of Medicine 2001); real half-lives run 1.5–9.5 h between people.

After 5 hours about 24 mg of the 47 are still in you, and after 10 hours about 12. The line crosses 25 mg at 4 h 33 min. The curve assumes an average adult. People have been measured with half-lives anywhere from 1.5 to 9.5 hours (Institute of Medicine, 2001), and on the slowest of those the same cup would still leave about 34 mg at the 4 h 33 min mark.

01Last black tea by bedtime

Bedtime	Finish by (25 mg left)	Left at lights out if finished 6 h before	Left if finished 3 h before
9:00 pm	4:25 pm	20 mg	31 mg
9:30 pm	4:55 pm	20 mg	31 mg
10:00 pm	5:25 pm	20 mg	31 mg
10:30 pm	5:55 pm	20 mg	31 mg
11:00 pm	6:25 pm	20 mg	31 mg
11:30 pm	6:55 pm	20 mg	31 mg
12:00 am	7:25 pm	20 mg	31 mg
12:30 am	7:55 pm	20 mg	31 mg
1:00 am	8:25 pm	20 mg	31 mg

Finish-by times are rounded down to 5 minutes. 25 mg is this site's planning line, not a clinical threshold. Every drink at once: caffeine cutoff grid.

02Why one mug of black tea is not one number

The USDA averages 41 samples of brewed black tea to 20 mg per 100 g. A cup of 237 g works out to 47 mg, the same as the headline. Stretch it to a 12 fl oz mug and the same data gives 71 mg, and the FDA independently lists black tea at 71 mg per 12 fl oz. Two agencies landing on one figure is rare for a plant product.

How the tea is made moves the number too. A 2025 lab study in Foods brewed six tea types at 80 and 100 degrees C for 2, 5 and 10 minutes and reported that, in black tea, cup-size bags gave 39 mg per 200 mL against 24 mg for loose leaf. Longer brewing raised caffeine in the infusion. Those are lab conditions, not your desk, so take the direction and ignore false precision.

Bottled tea runs thinner by the ounce but not by the bottle. USDA lists ready-to-drink black tea at 11 mg per 100 g, which makes a 16 fl oz bottle about 52 mg, more than a home cup. A sweetened lemon tea can is lower, near 24 mg. Decaf is not zero: about 2 mg a cup by the same arithmetic, and the FDA says decaffeinated teas still contain some caffeine.

Serving or brew	Caffeine	Basis	Source
8 fl oz black tea (headline figure)	47 (25–48)	Typical figure from the Mayo Clinic caffeine chart, as stored in the site data	Mayo Clinic
8 fl oz brewed, 237 g	47	20 mg per 100 g, 41 data points; multiplied by 237 g	USDA FDC 173227
12 fl oz brewed, 355 g	71	Same USDA figure multiplied by 355.2 g (12 × 29.6 g per fl oz)	USDA FDC 173227
12 fl oz black tea	71	FDA consumer update, per 12 fl oz serving	FDA
16 fl oz brewed, 474 g	95	Same USDA figure multiplied by 473.6 g (16 × 29.6 g per fl oz)	USDA FDC 173227
200 mL from a cup-size tea bag	39 ± 9.9	Lab measurement, mean ± SD; brewed at 80 and 100 °C for 2, 5 and 10 min	Foods 2025
200 mL from loose leaf	24 ± 7.2	Same lab study, black tea loose leaf	Foods 2025
16 fl oz bottled, ready to drink	52	11 mg per 100 g; multiplied by the 473 g USDA lists for 16 fl oz	USDA FDC 174144
8 fl oz decaffeinated brewed	2	1 mg per 100 g; multiplied by 237 g	USDA FDC 174871

Per-serving values from USDA mg per 100 g are calculated by DozeClock using the USDA gram weight of a fluid ounce (29.6 g) or the listed portion; they are not measured values for those cup sizes. The Foods 2025 figures are copied from the study abstract and describe lab brews.

03Black tea next to the three nearest drinks

Drink	Serving	mg	Hours to under 25 mg	Finish by for an 11 pm bed
Espresso	1 fl oz (30 ml) shot	64	6 h 47 min	4:10 pm
Instant coffee	8 fl oz (237 ml)	62	6 h 33 min	4:25 pm
Black tea	8 fl oz (237 ml)	47	4 h 33 min	6:25 pm
Cola	12 fl oz (355 ml) can	33	2 h	8:55 pm

mg per serving, not per ounce: servings differ in size · nearest = smallest difference in mg per serving (data/caffeine.json)

Cola holds 33 mg in a 12 fl oz can, but per fluid ounce black tea carries about twice as much (5.9 mg against 2.8). Instant coffee sits 15 mg above at 62 mg, and green tea about 19 mg below. If you are picking a drink for the evening, the swap that matters most is size: a 16 fl oz mug of black tea is about 95 mg, level with a standard cup of brewed coffee.

Sources

Caffeine content for coffee, tea, soda and more — Mayo Clinic (figure stored in DozeClock data; page not re-fetched on 2026-10-05)
FoodData Central: Beverages, tea, black, brewed, prepared with tap water (SR Legacy, FDC ID 173227; also 174144 and 174871) — U.S. Department of Agriculture
Spilling the Beans: How Much Caffeine is Too Much? — U.S. Food and Drug Administration
Determination of L-Theanine and Caffeine Contents in Tea Infusions with Different Fermentation Degrees and Brewing Conditions Using the Chromatographic Method — Foods (Basel), 2025
Caffeine for the Sustainment of Mental Task Performance: Formulations for Military Operations — National Academies Press, 2001