5.2 US Customary Units
Key Takeaways
- US customary length uses inches, feet, yards, and miles with relationships 1 ft = 12 in, 1 yd = 3 ft = 36 in, and 1 mile = 5,280 ft; conversions within this family are not powers of 10, so you must multiply or divide by the stated factor.
- US customary weight uses ounces, pounds, and tons where 1 lb = 16 oz (NOT 12 — a frequent trap) and 1 ton = 2,000 lb; volume uses fluid ounces, cups, pints, quarts, and gallons linked by 1 gal = 4 qt = 8 pt = 16 cups = 128 fl oz.
- Household measures common in home nursing include 1 cup = 8 fl oz, 1 tbsp = 3 tsp ≈ 15 mL, and 1 tsp ≈ 5 mL; these bridge to metric for patient education and home dosing.
- Multi-step conversions (e.g., gallons to cups, or pounds-and-ounces to total ounces) require chaining factors — write each step and cancel units to avoid arithmetic errors.
5.2 US Customary Units
Quick Answer: The US customary system is the everyday American system of inches, feet, pounds, cups, and gallons. Unlike the metric system, the relationships are not powers of 10 — 12 inches in a foot, 16 ounces in a pound, 3 teaspoons in a tablespoon — so you convert by multiplying or dividing by the stated factor, often in multiple steps. Nurses need this system for patient education, household dosing instructions, and for reading older intake/output records.
Length
| Relationship | Useful fact |
|---|---|
| 1 ft = 12 in | foot/inch conversions |
| 1 yd = 3 ft = 36 in | fabric and short distances |
| 1 mile = 5,280 ft | walking/exercise distance |
Worked Example 1: 3 qt = ___ cups
Use the chain: 1 qt = 2 pt and 1 pt = 2 cups, so 1 qt = 4 cups.
3 qt × 4 cups/qt = 12 cups.
Alternatively, from the master volume ladder 1 gal = 4 qt = 8 pt = 16 cups, one quart is one-fourth of a gallon, so it holds 16 ÷ 4 = 4 cups. Same answer.
Worked Example 2: 2 lb 6 oz = ___ oz
This is a mixed-unit conversion. Convert the pounds to ounces, then add the remaining ounces.
2 lb × 16 oz/lb = 32 oz.
32 oz + 6 oz = 38 oz.
Weight
| Relationship | Useful fact |
|---|---|
| 1 lb = 16 oz | the most commonly missed US conversion |
| 1 ton = 2,000 lb | large-scale weight, rarely used in nursing |
Volume
| Relationship | Useful fact |
|---|---|
| 1 gal = 4 qt = 8 pt = 16 cups = 128 fl oz | master volume ladder |
| 1 qt = 2 pt = 4 cups = 32 fl oz | |
| 1 pt = 2 cups = 16 fl oz | |
| 1 cup = 8 fl oz | household cooking and oral intake |
Household / Nursing Equivalents
Patients dose themselves at home with kitchen spoons and measuring cups, so nurses must teach approximate equivalents:
| Household | Approximate metric |
|---|---|
| 1 tsp | ≈ 5 mL |
| 1 tbsp = 3 tsp | ≈ 15 mL |
| 1 cup = 8 fl oz | ≈ 240 mL |
| 1 fl oz | ≈ 30 mL |
These are approximate — a household teaspoon can vary from 3 to 7 mL depending on the spoon. That is why hospitals use calibrated oral syringes, not kitchen spoons, for pediatric and high-risk medications. Teach families to use the dosing syringe that comes with the medication.
Worked Example 3: 2 gal = ___ fl oz
Using the master ladder, 1 gal = 128 fl oz.
2 gal × 128 fl oz/gal = 256 fl oz.
Worked Example 4: A patient records 1 cup of water, 1 pint of juice, and 6 fl oz of milk at lunch. What is the total intake in fl oz?
1 cup = 8 fl oz. 1 pt = 16 fl oz. 6 fl oz = 6 fl oz.
8 + 16 + 6 = 30 fl oz.
Worked Example 5: 5 yd = ___ in
Chain the factors: 1 yd = 3 ft and 1 ft = 12 in, so 1 yd = 36 in.
5 yd × 36 in/yd = 180 in.
Common Traps
- Treating ounces-per-pound as 12. The 12 comes from inches-per-foot, not ounces-per-pound. One pound is 16 ounces. Mixing these up is the single most common US-customary error on nursing math exams.
- Confusing weight ounces with fluid ounces. "oz" by itself is ambiguous; in a nursing context, weight uses oz (weight) and volume uses fl oz. They are not interchangeable — 8 fl oz of water weighs about 8 oz, but 8 fl oz of syrup weighs far more. When a problem states "8 oz," check whether it is weight or volume from context.
- Skipping the master volume ladder. Students try to memorize each pair separately. Instead, anchor on 1 gal = 128 fl oz and derive: 128 ÷ 4 = 32 fl oz per quart, 32 ÷ 2 = 16 fl oz per pint, 16 ÷ 2 = 8 fl oz per cup.
- Forgetting that cups are 8 fl oz, not 6 or 10. A "cup of coffee" is not necessarily 8 fl oz (mugs vary), but a measuring cup is exactly 8 fl oz by US definition.
- Mixing metric and household spoons. A kitchen tablespoon is approximately 15 mL but can be 12–18 mL. Always specify "measuring spoon" when teaching patients.
Strategy: Cancel Units
For multi-step conversions, write the starting value with its unit, then multiply by a fraction equal to 1 (the conversion factor) chosen so the unwanted unit cancels. Example for 5 yd → in:
5 yd × (3 ft / 1 yd) × (12 in / 1 ft) = 5 × 3 × 12 in = 180 in.
The yards cancel the yards, the feet cancel the feet, and you are left with inches. This unit-cancellation method, also called dimensional analysis, is the safest way to do every nursing calculation because the units tell you whether your setup is right before you ever touch a calculator.
A patient's lunch intake: 1 cup of broth, 4 fl oz of gelatin water, and half a pint of juice. What is the total intake in fluid ounces? (1 pt = 16 fl oz, 1 cup = 8 fl oz)
A newborn weighs 8 lb 0 oz at birth. How many total ounces is this? (1 lb = 16 oz)