How much does water weigh?

Update 2009-08-07: Now the slightly lower density of water at room temperature is taken into account, improving the answer by 0.3%.

Is this a silly question? Not at all, if you're using either American or Imperial measurements.

I just ran across two web pages that say one cup of water weighs 8 ounces (oz). To protect the guilty, I won't mention the sites except to say that one is from a physicist and the other from a cooking magazine, both of whom should know better—though from different perspectives—than to publish this incorrect information. Let's do the math:

By definition, 1 liter (L) holds 1000 cubic cm (cm³, or cc) of volume, thus 1 milliliter (mL) is the same as 1 cc in volume. The weight of water was also initially defined as being 1 kilogram (kg) per liter, but that definition has been discarded since the density of water changes with temperature and atmospheric pressure (if you're curious, this PDF outlines some factors to consider when measuring water in a lab). Taking this into account, the following calculations are based on data at room temperature (25 °C/77 °F) and 1 atmosphere (basically, air pressure at sea level). Tap water is slightly denser than pure water, but the difference is negligible at room temperature. Under these conditions, water has a density of 997.13 kg/m³. For the calculations, we'll also need a few conversion factors:

1 m = 100 cm
1 kg = 1000 g
1 L = 1.0567 US quarts (qt)
1 qt = 4 cups
1 lb = 16 oz = 453.59 g

Just to be explicitly clear, all non-metric weights in this post are expressed in avoirdupois units. This is the ordinary weight system used in the US (as opposed to troy units, which are used for weighing precious metals). Since avoirdupois is the system in common usage, I think it'll be a bit cleaner to omit this extra notation in the equations. Now we calculate,

(997.13 kg/m³) × (1000 g/kg) × (1 m/100 cm)³ × (1000 cm³/L) = 997.13 g/L

Converting to US units,

(997.13 g/L) × (1 lb/453.59 g) × (1 L/1.0567 qt) = 2.0803 lb/qt

Finally, converting to cups,

(2.0803 lb/qt) × (16 oz/lb) × (1 qt/4 cups) = 8.3214 oz/cup (US)

In the US customary measurement system, then, one cup of water, which is 8 fluid ounces (fl. oz) in volume, is actually 8.3214 oz in weight. Is it close enough? I don't think so. If you were measuring water on a digital scale that displays 1/8-oz increments, the nearest reading would be 8-3/8 oz. For scales with 1/4-oz resolution, the nearest reading would be 8-1/4 oz.

Based on the above, we can make some handy tables for various volumes. Let's start with metric figures first:

Weight of water at 25 °C (77 °F) at sea level.

Metric Volume	US Volume	Imperial Volume	Metric Weight	Avoirdupois Weight
1 milliliter (= 1 cc or cm3)	0.03381 fl oz	0.03520 fl oz	0.9971 g	0.03517 oz
1 liter	1.057 qt	0.8799 qt	0.9971 kg	2.198 lb
1 cubic meter	264.2 gal	220.0 gal	997.1 kg	2198 lb

In US units:

* Based on these defined (i.e., exact) figures: 1 US liquid gallon = 231 in³ and 1 inch = 2.54 cm. See the Wikipedia links below for source data.

US Customary Volume	Multiplier (exact)	Metric Volume*	Avoirdupois Weight	Metric Weight
1 teaspoon	N/A	4.929 mL	0.1734 oz	4.915 g
1 tablespoon	= 3 tsp	14.79 mL	0.5201 oz	14.74 g
1 fluid ounce	= 2 tbsp	29.57 mL	1.040 oz	29.49 g
1 cup	= 8 fl oz	236.6 mL	8.321 oz	235.9 g
1 pint	= 2 cup	473.2 mL	16.64 oz (1.040 lb)	471.8 g
1 quart	= 2 pt	0.9464 L	33.29 oz (2.080 lb)	0.9436 kg
1 gallon	= 4 qt	3.785 L	8.321 lb	3.775 kg
1 cubic inch	N/A	16.39 mL	0.5764 oz	16.34 g
1 cubic foot	= 1728 in3	28.32 L	62.25 lb	28.24 kg
1 cubic yard	= 27 ft3	764.6 L	1681 lb	762.4 kg

Let's pause for a moment and think about terminology a bit. It seems that some people have trouble distinguishing between how weight and volume are defined. What's the difference? Simply,

Weight = how heavy
Volume = how big

It's important to remember that different substances filled to the same size (volume) can have different weights. To use a classic example, a bucket full of feathers will be lighter in weight than the same bucket filled with rocks. The same principle applies to liquids in the kitchen as well. A cup (that is, 8 fluid ounces) of oil will weigh less than a cup of water.

Speaking of fluid ounces, what do you think happens when the name of a unit normally associated with weight (oz) gets included in a unit for measuring volume (fl. oz)? Constant confusion, that's what. The nomenclature makes for a huge mess, but it's what we're stuck with.

It also doesn't help matters that the US measures are colloquially called "English" or "British" units, even though the official US and Imperial systems differ significantly when it comes to measuring volume. As it happens, 1 Imperial fl. oz equals 0.96076 US fl. oz, so 8 Imperial fl. oz of water is equivalent to 7.9948 oz by weight, which I believe is actually close enough to 8 oz for cooking-measurement purposes.

Further, not only do the measurements differ between US customary and Imperial units, so do the multipliers. An Imperial pint, for example, is 20 Imperial fl oz, whereas a US pint is 16 US fl oz. So a pint of water weighs either 16.643 oz (US) or 19.987 oz (Imperial). Not quite "a pint's a pound the world around," there. At any rate, we can also calculate a table for Imperial units:

* Based on these defined (i.e., exact) figures: 1 Imperial gallon = 4.54609 L & 1 Imp. tbsp = 5/8 Imp. fl. oz. See the Wikipedia links below for source data.

Imperial (UK) Volume	Multiplier (exact)	Metric Volume*	Avoirdupois Weight	Metric Weight
1 teaspoon	N/A	5.919 mL	0.2082 oz	5.902 g
1 dessertspoon	= 2 tsp	11.84 mL	0.4164 oz	11.80 g
1 tablespoon	= 3 tsp	17.76 mL	0.6246 oz	17.71 g
1 fluid ounce	= 1.6 tbsp	28.41 mL	0.9994 oz	28.33 g
1 cup	= 10 fl oz	284.1 mL	9.994 oz	283.3 g
1 pint	= 2 cup	568.3 mL	19.99 oz (1.249 lb)	566.6 g
1 quart	= 2 pt	1.137 L	39.97 oz (2.498 lb)	1.133 kg
1 gallon	= 4 qt	4.546 L	9.994 lb	4.533 kg
Cubic inch, foot, and yard figures are the same as US measures.

Notice how close an Imperial gallon of water comes to weighing 10 lbs. That's because, similar to the original kg/L equivalence, the Imperial gallon was defined as the volume occupied by 10 lbs of distilled water, but at a lower temperature and slightly higher pressure than what I used here (specifically, 62 °F and 30 inches of mercury, which is about 1.0025 atm).

Conversely, we can also figure volumes based on a starting weight, again under the condition of 25 °C (77 °F) at sea level:

Weight of water	US Volume	Imperial Volume	Metric Volume
1 gram	0.03391 fl oz	0.03530 fl oz	1.003 mL
1 kilogram	33.91 fl oz (1.060 qt)	35.30 fl oz (0.8824 qt)	1.003 L
1 avoirdupois ounce	0.9614 fl oz	1.001 fl oz	28.43 mL
1 avoirdupois pound	15.38 fl oz (1.923 cup)	16.01 fl oz (1.601 cup)	454.9 mL

You know, this post started as a rant but turned into a bunch of conversion tables. How did that happen? I think I'll stop here before I get all worked up over the distinction within the US system itself of liquid quarts vs. dry quarts or how the customary measures used in cooking differ from the FDA-regulated measures used for food nutrition labeling. Makes me wish all of our recipes would be published in metric, but I know I'll never see the day.

Oh, if you've read this far (thank you, really), you may now be wondering why I haven't yet provided a conversion table between US and Imperial volumes or between cubed lengths and volumes. The reason is because this post is about the weight of water, expressed in multiple units, and not about converting a volume from one unit to another. Besides, the existing tables already contain enough information, in the form of metric volumes or weights, for you to derive these conversions yourself—or, for a more direct route, you can use any of the calculators available online. You can also check with Wikipedia for conversion tables and unit definitions:

• Comparison of US & Imperial Units
• Conversion of Units
• US Customary Units
• Fluid Ounce

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Texas caviar

Update: For a simplified version of this recipe, using canned products, go here.

A belated Happy New Year to everyone. Around these parts, black-eyed peas are traditionally eaten on New Year's Day for good luck. Given how the last half of 2008 went, I think 2009 will need all the help it can get. While one can simply prepare black-eyed peas (which are also called cowpeas, but are actually beans) with just a few flavorings, I like to mix it with other ingredients into a bean salsa called Texas Caviar. As far as I know, there is no such thing as a canonical Texas Caviar, and a search on the Internet shows that recipes are certainly all over the map. My version is based on the recipe from p. 280 of American Home Cooking by Bill and Cheryl Jamison, but in reality I'm still tweaking it. The recipe below, then, is not what I had just made for this past New Year's, but what I'm planning for the next time around. (It's pretty close, though—the base ingredients are in good shape, I just need to get the after-cooking yields straightened out, as you'll see.)

INGREDIENTS

Beans and Corn

6 oz.	Black beans (dried, about 3/4 cup), plus
	3 cups water and 1 tsp table salt
8 oz.	Black-eyed peas, fresh or frozen, plus
	4 cups water and 1 tsp table salt
3/4 cup	Yellow corn kernels

Aromatic Vegetables

1 small	Red bell pepper (about 2/3 cup diced)
3 stalks	Celery (about 2/3 cup diced)
1 small	Onion (about 1/2 cup diced)
1 medium	Jalapeno chile, fresh or pickled

Dressing and Seasoning

1/4 tsp	Ground cumin, or
	1/4 heaping tsp whole cumin seeds (preferred)
2 medium	Garlic cloves
1 tsp	Coarsely ground black pepper, or more to taste
1/2 tsp	Table salt, or more to taste
1/4 cup	Apple cider vinegar
1/2 cup	Neutral-tasting vegetable oil

INSTRUCTIONS

COOK THE BEANS AND CORN

For beans, there are evidently some pretty strongly-held opinions on soaking methods (long soak, quick soak, or no soak) and when to add salt during cooking. From all I've read, none of it really matters, as soaking vs. not-soaking merely changes the cooking time, and, contrary to much handed-down advice, adding salt early doesn't toughen the beans. But if you disagree with this and have another way that you have (and can be) relied upon, then hey, do whatever works for you; I'm not going to argue.

Black beans

• Spread the beans out on a flat surface and pick out any pebbles, loose skins, and broken bits and discard.
  
• Place the beans in a cooking pot and fill with water. Pick out any floaters and discard.
  
• If you are able to plan ahead, soak the beans overnight (8 to 12 hours) before cooking. If not, it's no problem to cook the beans right away, it'll just take more time.
  
• If you've let the beans soak, drain the soaking water.
  
• Fill the pot with 3 cups water and place on a stove.
  
• Bring the water to boil over High heat, then reduce heat to between Low and Med-Low so that the water is simmering. Skim off the foam, add 1 tsp salt, then place a lid on the pot and let simmer until the texture is to your liking. Check on the beans starting at about 30 minutes. Soaked beans that are fresh will cook the quickest, while old and/or unsoaked beans will take longer.

Black-eyed peas

• Place the beans in a cooking pot (approx. 3 qt. capacity) and fill with water. Pick out the floaters, loose skins, and broken bits and discard. Drain water and rinse/drain a few more times until the water is clear.
  
• Now fill the pot with 4 cups of water and place on a stove.
  
• Bring the water to boil over High heat, then reduce heat to between Low and Med-Low so that the water is simmering. Skim off the foam, add 1 tsp salt, then place a lid on the pot and let simmer for until bean texture is to your liking. As with the black beans, start checking at about 30 minutes.

Corn

• You can use kernels cut off a cob or frozen kernels. Here in the middle of winter, I just zapped some corn straight from the freezer into the microwave on high power for about 2 minutes.
  
• Place the cooked corn into a mixing bowl (6 qts is a good size) to combine with other ingredients later.

PREPARE THE VEGETABLES

Red bell pepper

• Remove and discard the stem, core, and seeds. Then trim away the white portions of the ribs.
  
• Cut the pepper into 1/4" square pieces (technically, a small dice).
  
• Place the pepper dices into the mixing bowl with the corn.

Celery

• Either wash the stalks under running water or swish them in a bowl of water to make sure that all the dirt and grit have been removed.
  
• Remove the leaves if there are any. Cut off approx. the bottom 1" of each stalk. Either discard these trimmings or save them for making stock another time.
  
• Cut each stalk into 1/4" squares.
  
• Place the celery dices into the mixing bowl with the corn and bell pepper.

Onion

• Cut off the very top and bottom portions of the onion. Remove the papery skin. Discard trimmings or save for stock.
  
• Chop the onion into 1/4" squares.
  
• Place the onion dices into the mixing bowl with the corn, bell pepper, and celery.

Jalapeno

• I recommend handling hot chiles (such as jalapenos) with gloves. Disposable or surgical gloves are nice for this purpose. Otherwise, at least be very careful not to touch sensitive parts of your body, such as your eyes or any cut skin, if you've handled hot chiles with bare hands.
  
• Remove and discard the stem. You may keep or discard the core and seeds depending on your desired level or heat. Chiles with removed core and seeds will be less hot.
  
• Cut the jalapeno into 1/8" squares. For you fancy-schmancy types, this is a brunoise.
  
• Place the jalapeno pieces into the mixing bowl with the previous ingredients. Stir to mix.

PREPARE THE DRESSING

Ideally, you'd like to have a good oil/acid emulsion, and the best emulsions are achieved by running a blender with vinegar in its pitcher while slowly drizzling oil into it. However, this is potentially a messy operation (I'll get to that in a bit). The next-best method as far as emulsion quality goes is to keep whisking the vinegar by hand while drizzling oil into it. But you need three arms to whisk, drizzle, and hold the bowl! Well, you can put a wet towel on the counter and set the bowl on that to keep it stationary. But if you're just not up to that, simply putting the oil, vinegar, and seasonings together and whisking them all at once works, too; the emulsion won't hold, but the taste really doesn't suffer much.

I should also mention here that classic oil:acid ratio is 3:1 for vinaigrettes, but I think this recipe has enough sweet ingredients in it (corn, bell pepper, and celery) that a stronger acidic bite in the dressing serves to balance out the flavors better.

• If using ground cumin, place it into a mixing vessel—either a small mixing bowl or a blender's pitcher. Otherwise, put the whole seeds into a small skillet and place it on the stove. Toast the seeds over medium heat until fragrant. Stir the seeds or shake the pan to prevent burning. After toasting, grind the seeds with a mortar and pestle, spice grinder, or coffee mill, then place the ground seeds into the mixing vessel.
  
• Peel and mince the garlic cloves, then place into the mixing vessel.
  
• Add the vinegar to a mixing vessel, either a small mixing bowl or a blender's pitcher.
  
• Add the salt and pepper to the mixing vessel. You can adjust for taste later.
  
• Add the vinegar to the mixing vessel and combine with the oil as mentioned above, either by running the blender or whisking manually.

Now, about the blender mess. I don't know how common this is, but evidently the "small" hole in my blender's lid is still big enough to allow vinegar and oil to spew out of it when the blender is running. Result? Oil and vinegar droplets all over the countertop and upper cabinets. I'm thinking that maybe next time I should put a funnel over the lid to cover up most of this opening. That'll probably help with the drizzling, too. Or use a stick blender in a cup. I just have to remember to make sure the blades are fully immersed before turning it on, otherwise I'll have another mess. Or, maybe I could practice first. With water. Outdoors.

COMBINE BEANS AND DRESSING

• Pour the cooked black beans and black-eyed peas into a colander and drain completely.
  
• Empty the colander into the mixing bowl that already contains the corn, celery, bell pepper, onion, and jalapeno.
  
• Pour the oil/vinegar mixture over the cooked beans and stir with a spatula or wooden spoon to mix. Adjust salt and pepper to taste.
  
• You can serve this immediately, but it's better to let it rest in a refrigerator for at least a few hours for the flavors to combine.

YIELDS

Measurements taken while making this year's batch:

• 8 oz. raw black-eyed peas measure approx. 2 cups (!?)
  
• 8 oz. raw black-eyed peas weigh 9.6 oz. and measure approx. 1-2/3 cups by volume after cooking. This makes no sense. Either the before or after cooking volumetric measure is wrong, or some of these beans mysteriously disappeared during cooking.
  
• 8 oz. dried black beans weigh 1 lb. and measure 2 cups after soaking but before cooking.
  
• 8 oz. dried black beans weigh 1 lb. 2 oz. and measure approx. 3 cups by volume after cooking. This is somewhat in conflict with my earlier measurements, which showed that 8 oz. dried black beans yield between 2-2/3 and 2-3/4 cups after cooking. Also, we can see that there is additional water absorption during cooking, but I'm surprised that the volume increased by 50%. I'll have to see where this data anomaly comes from.

Adding all the ingredients together and considering the uncertainties, this recipe should make between 6 to 6.5 cups of finished Texas Caviar.

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