Caffeine per ounce is a concentration: the milligrams of caffeine in a stated fluid-ounce serving. It can be useful when comparing small-volume drinks, but it does not by itself show the total caffeine consumed or determine the rate at which caffeine is absorbed. Recipes, products, and container sizes vary, so a category name such as "coffee" or "energy drink" is not a fixed caffeine value. The U.S. Food and Drug Administration (FDA) makes the same practical point: caffeine amounts in specific products and container sizes can vary. FDA
The calculation
Use this simple calculation for a drink with a stated caffeine amount and volume:
caffeine concentration (mg/oz) = caffeine in the stated serving (mg) ÷ stated serving volume (fl oz)
The table below uses the reference values in Mayo Clinic's February 2025 comparison. The final column shows the arithmetic, rounded to one decimal place. These are representative prepared-drink reference points, not measurements of every product in each category. Mayo Clinic
| Reference item | Stated serving volume | Stated caffeine | Calculation | Caffeine per ounce | Variability note |
|---|---|---|---|---|---|
| Brewed coffee | 8 fl oz | 96 mg | 96 ÷ 8 | 12.0 mg/oz | Coffee caffeine can vary with factors including growing and brewing conditions. Mayo Clinic |
| Espresso | 1 fl oz | 63 mg | 63 ÷ 1 | 63.0 mg/oz | This is a reference serving, not a value for every espresso recipe or café shot. Mayo Clinic |
| Instant coffee | 8 fl oz | 62 mg | 62 ÷ 8 | 7.8 mg/oz | The figure is a reference value for the stated prepared serving. Mayo Clinic |
| Brewed black tea | 8 fl oz | 48 mg | 48 ÷ 8 | 6.0 mg/oz | Tea caffeine also varies with growing and brewing conditions. Mayo Clinic |
| Brewed green tea | 8 fl oz | 29 mg | 29 ÷ 8 | 3.6 mg/oz | The figure is a reference value for the stated prepared serving. Mayo Clinic |
| Energy drink | 8 fl oz | 79 mg | 79 ÷ 8 | 9.9 mg/oz | This is a generic reference entry; check the current label of a particular product. Mayo Clinic |
| Energy shot | 2 fl oz | 200 mg | 200 ÷ 2 | 100.0 mg/oz | This is a generic reference entry, not a claim about every shot or a product ranking. Mayo Clinic |
Within this limited reference table, the energy-shot entry has the highest concentration and the espresso entry is also much more concentrated than an 8-ounce brewed coffee. That does not establish a universal "strongest drink." A different product, recipe, or serving can change both the milligrams and the mg/oz result. Mayo Clinic
Concentration and total caffeine answer different questions
A 1-ounce espresso reference serving in the table has 63 mg of caffeine. An 8-ounce brewed-coffee reference serving has 96 mg. Espresso is more concentrated in that comparison, while the stated brewed-coffee serving contains more caffeine overall. Use mg/oz when the practical question is about a small-volume serving; use total milligrams per finished serving when adding up intake. Mayo Clinic
The same distinction matters for a concentrate. A concentrate should be treated separately from a ready-to-drink beverage because the useful comparison is the caffeine and volume in the finished serving actually consumed. Do not assign one general mg/oz value to "cold-brew concentrate" or carry a concentrate's figure over to every diluted drink. Start with the product's current caffeine statement, if available, and the stated serving or preparation information. The FDA notes that product caffeine amounts and container sizes vary; it also encourages consumers to ask retailers when caffeine information is not available. FDA
Pure or highly concentrated caffeine is a different category again, not a beverage comparison. The FDA warns that these products can have serious health consequences and says toxic effects can be observed with rapid consumption of around 1,200 mg of caffeine. FDA
Putting the numbers in health context
For most adults, the FDA has cited 400 mg a day as an amount not generally associated with negative effects. This is qualified public-health guidance, not a personalized limit: the FDA notes wide variation in caffeine sensitivity and in how quickly people eliminate caffeine. It also advises speaking with a health care professional about caffeine if you are pregnant, trying to become pregnant, breastfeeding, concerned about a medical condition or medication, or experiencing symptoms you think may be related to caffeine. FDA
A label or published nutrition statement is the best starting point for a specific packaged drink. Record the stated caffeine and the stated serving volume, calculate mg/oz if concentration matters, then count the full serving's milligrams alongside caffeine from coffee, tea, energy drinks, foods, medicines, and other sources. FDA