approximating points in a grocery store

I got a question via email about how to approximate the points total for an item (which sounds pretty useful) when you’re “on the go” in the grocery store and don’t want to break out a slide rule.

Could you please tell me if there is a way to figure out the approx
points without having a calculator. Is there some kind of guide line
that I can use when going to the grocery store. Thanx Linda

Sure!  That’s a great question.  There’s only 3 parts of the formula
(calories, fat, fiber) and there’s a couple of
simplifying/approximating things we can do.

Just calories
The most inaccurate but easiest approximation would be to look at just
the calories.  Since it’s the number of calories divided by 50, we can
either divide by 50, or shift the decimal place over 2 and double, or
whatever you find easiest – when picking the number to represent, just
round to the nearest 50.  If the calories is 180, that’s about 200,
which would be 4 points.  If the calories are 110, that’s about 100, or
2 points, etc.

Calories + Fat
If we want a little more accuracy (and are willing to the extra work),
then the next piece to add in is fat.  Since a lot of WW-friendly
things are pretty low in fat, we can generally ignore the fat content,
but it’s the next piece if we’re trying to get a little more accuracy. 
The actual formula says to divide the fat grams by 12, but that’s not a
very easy number to divide by, so we’ll divide by 10 instead, which is
definitely much easier.  5 fat grams?  That’s + 0.5 points to the
total.  15 fat grams?  + 1.5 points.  20 fat grams?  + 2.0 points,
etc.  As with calories, just round to something close since we’re
approximating.  8 fat grams?  That’s close to 10, which is +1.0 points.

All 3
Fiber is the last thing to potentially add in to our approximation, but
it can only count up to 4 grams, and even then it only saves you 0.8
points.  The easiest way to factor this in is to see if the fiber grams
are 3 or higher – if they are, subtract off a point, otherwise, don’t.

Just for fun, let’s compare some numbers to get a feel for how “off” these approximations are.

Item: 200 calories, 6 fat grams, 3 fiber grams

Actual: 4 + 0.5 – 0.6 = 3.9 which rounds to 4 points

Just Calories approximation:  200 calories / 50 = 4 points

Calories + Fat = 4 points + 0.5 = 4.5 points (maybe round to 5)

All 3 = 4 + 0.5 – 1 = 3.5 (maybe round to 4)

I’d guess that for most things, these approximations would be
reasonable – calories are almost always going to be a pretty good
estimate by themselves, so in a real rush, just dividing calories by 50
is at least within shouting distance of accurate.

Thanks, Linda!  Great question.