
Medicine Calculations for the Mathematically ChallengedWe'll first start with a math lesson: When calculating any dosage, you will need the following:
Now that you have all the information you need to calculate your rat's dosage, you need to assemble it in a way that will result in a number that "makes sense." What I do, to keep myself straight, is to start with the actual recommended dosage. I do this because I have to start somewhere and it seems like a good place to start :o) Using the numbers from above (so as to avoid confusion): I want my "kg" numbers to be diagonal from one another:
I also want my "mg" numbers to be diagonal from one another:
And you can do that for the same reason why you can assume that X ÷ X equals 1. 2 ÷ 2 = 1 and 2002 ÷ 2002 equals 1. And every number (whether a number or whether it's denoted by X) divided by the same number will always equal 1. Think of the units "mg" and "kg" as X and it should make sense. But there aren't two instances of "ml" are there? There doesn't have to be. When you're calculating the amount of medicine you are to give your rat, you want to know how much, in mls (or ccs) to give your rat. When you get done multiplying and dividing all the numbers, that's what you end up getting: A number that "makes sense" that tells you how many mls (or ccs) your rat needs:
Now look back up at the two examples where you were cancelling the kgs and the mgs out: When you looked at just the kilograms you ended up with 0.454 and no units. When you looked at just the milligrams, you ended up with ½ and no units. 0.454 divided by 2 is 0.227 (no units). Multiply that by your 1 ml and now you have 0.227 ml. And that's the same number you'd get if you were to take all the numbers in the "setup" (above), multiple all the numbers across the top row and divide by all the numbers across the bottom row. How'bout some examples:
The recommended dosage is 0.5 mg/kg to be given every 12 hours. Your rat weighs 13 ozs. You have a 5 mg pill. Step 1: Don't panic! Step 2: The recommended dosage is 0.5 mg/kg so you know the rat's weight can't be in ounces. The first thing you do is multiply 13 ounces by 28 grams per ounce to find that your rat weighs approximately 364 grams. Step 3: But wait! The recommended dosage is 0.5 mg/kg and you just found your rat's weight in grams so you divide 364 by 1000 to get 0.364 kg. Step 4: So far, so good but you have a 5 mg pill. It's a pretty small thing all things considered so even if you just stop here and do the calculations (0.5 mg/kg X 0.364 kg  the kg cancel out and you are left with 0.182 mg) you realize that there is no reasonable way to cut out 0.182nd of a pill. So you say, "OK, I should add some liquid" and thinking that 5 ÷ 5 calculates to a nice easy number, you add 5 ml of water to your calculations (and you crush the pill into a fine powder and mix it with 5 ml of liquid). Step 5: Now you're ready to go. Set it up:
Step 6: ...and do the math:
The recommended dosage, which you obtained from the RMCA is 10mg/lb BID, PO. Your rat weighs 500 grams. From the box you find that each ml contains 50 mg of amoxicillin when mixed. Step 1: Don't panic! Step 2: I tricked you! Most dosages, when you look them up in the literature, are given in mg/kg but there are some sources that, geared toward an American metricsystem hating audience, will list the dosage in mg/lb. What to do? Well, you can convert the dosage into mg/kg: ...or you can convert your rat's weight to lbs:
Step 3: Now you have a dosage in mg/lb, and you've figured out how many lbs a 500 gram rat is. You also already have a suspension so you needn't worry about creating one. That means every unit has a diagonal counterpart (except for the thing you're trying to find: the mls) so you're ready to go. Set it up:
Step 6: ...and do the math: Or if you figured out how many mg/kg 10 mg/lb was, you could do it this way:
tip: When navigating through the RatTails, clicking on the image that looks like the image above will take you back to this table of contents!
Disclaimer: There are many nonsarcastic accounts and tips on the web regarding rat care. This is not
one of them. These are merely accounts of our experiences with rats, our perceptions of these experiences, where we've failed
and where we've succeeded. These accounts are here for two purposes:
Remember! Your rat is not a science project, he is your friend!

