Well, I finally have a tomato this season which should make it to two pounds.

This fruit has been designated Big Zac (4.20 Diaz 2011) B-001, where:

Big Zac = variety name. Note that the original Big Zac tomato was a HYBRID. Consequently, many tomato experts urge using a filial designation (F1, F2, F3, etc.) to indicate then number of generations removed from the F1. In the world of giant tomato growing, however, selection of seed for saving is made essentially just on fruit size, so there will be (and IS) significant variation among other traits for giant tomatoes grown from Big Zac lineage. Of course for open pollinated varieties, this variation is much less of an issue.

4.20 = 4.20 lbs., the certified (as is the case with the parent of this tomato) or at least documented (with pictures on reliable scales and with a measuring tape, for example).

Diaz = Julian Diaz of Valparaiso, Indiana, United States of America – the grower of this contest-winning monster.

2011 = The year this tomato was grown.

B-001 = My tracking system, where “B” refers to Phase B of my giant tomato project (i.e., heavy pruning) and 001 refers to the first tomato tracked on this plant.

The parent of this 4.20 Diaz was Big Zac (3.59 Perry 2009). This in turn was grown from Big Zac (5.32 Timm 2008), but this lineage is not certain. So the tomato profiled here is probably a Big Zac (F6).

Once this tomato is harvested and weighed, it will receive it’s own designation. My prediction at this point is:

Big Zac (2.090 DT 2012)

So that is the predicted final weight, once I harvest and weigh the tomato (“DT” refers to Delectation of Tomatoes, although my last name, Thurber, would be entered for official public records). This prediction is based upon measurements which I’ve been taking, starting on 7-27-2012.

What measurements? I use vernier calipers and measure to the nearest millimeter (actually I interpolate and record one-tenths of mm, or microns).

So I would record this measurement as 51.7mm. Why calipers, when most people use tape measures? 1) Speed – depending upon how much vegetation is in the way, one measurement can take 10 seconds or so (I take lots of measurements…); 2) Injury – encircling a growing tomato with a tape measure is risky business – inadvisable unless you’re about ready to pick it anyhow; 3) Durability – this metal instrument has been used thousands of times and is still in excellent condition, though I have to wash it in soap and warm water on occasion.

Our **OBJECTIVE** is to estimate the weight of a tomato, in grams, based upon measurements taken with either a tape measure or with calipers. You can convert from metric to English at the very end, if desired. (Most official weights are recorded in pounds and decimal pounds to 2 or 3 digits).

First, let’s consider measuring **CIRCUMFERENCES** with a **TAPE MEASURE**

Which measurements to take? I take three measurements, in cm, for use in the formula for calculating the volume of an **ELLIPSOID**. Let’s orient the tomato first. Consider the stem end as the top or “North Pole” and the blossom end as the bottom or “South Pole”. Take three measurements as follows:

1) CC1 = Primary Axis = the longest circumference possible measured through both the stem end and the blossom end (poles) as shown (Scotch tape is very helpful in keeping the tape snug):

So CC1 = 35.9mm

2) CC2 = Vertical Axis = The circumference taken 90° from CC1 as shown:

So CC2 = 28.4

3) CC3 = Minor Axis = Longest circumference around the equator, at right angles to CC1 and CC2:

So CC3 = 37.4mm.

The basic formula for calculating the volume of an ellipsoid is:

V=4/3πabc, where a, b, and c are the 3 perpendicular radii of the ellipsoid. Since we measured circumferences with the tape measure and CC=2*π*r, we can simplify the formula for this purpose to:

V=0.016887*CC1*CC2*CC3 (in cubic centimeters, or cm^3)

In this example, V=0.016887*35.9*28.4*37.4 = 644 cm^3 (rounded)

This will give you an approximation of the volume of your tomato. The more bumps, crevices, lobes, fluting and other irregularities on your tomato, the less accurate this formula will be.

So you have the volume. How do convert this to weight? If we are talking about pure water, this would be easy, as 1 cm^3 of water weighs 1 gram. But tomatoes do not have the same density (specific gravity) as water. I’ve taken density measurements of more than 800 tomatoes (results needs to be published…), so I’ve got a pretty good handle on this variable.

Compared to water, the approximate density of

Green Tomatoes = 0.90

Ripe and ripening (at least 25% ripe) tomatoes = 0.95

These I’ve designated as **Density Adjustment Factors** (**DAF**)

Many growers have noted a slight shrinkage of tomatoes as they ripen, so they recommend picking “at first blush”. I’ve also measured this shrinkage several times, but have found that it is compensated for, at least, by higher fruit density. So I suggest picking tomatoes when they appear to be at least 25% ripe. In my experience, measurable shrinkage rarely occurs until tomatoes are at least 80% ripe.

So if you want your prize tomato to be as **heavy as possible**, harvest it when its approximately **75% ripe** in the early **morning** after a heavy rain or watering. If your tomato is ripening several days before the big event, pick it when it’s 25-50% ripe and store it at 50-60° with plenty of air circulation and cushioned. If the tomato is cracked, you might considered treating the wounds with a 10% bleach solution every couple of days. If cared for properly, competition tomatoes can be stored for up to two weeks.

So, assuming you pick your tomato at 25% or more ripe, the formula for calculating the weight, in grams, of a tomato from measured circumferences becomes:

W(weight)=0.016887*CC1*CC2*CC3*.95 or

**W=0.016043*CC1*CC2*CC3** (weight in grams)

For the green tomato shown, we would use 0.90 for the **DAF** instead, which would be 644*0.90 = 580 g. Actual weight was 572 g, so accurate to within 2%. Using this method, we can expect to be within 10% for most tomatoes. I routinely expect 5-15% lower weights for lobed tomatoes, especially if they are deeply lobed. Some tomatoes, depending largely upon variety, have some hollowness (air pockets) in their seed locules. This gives them a “spongy” feeling when gently squeezed, even before they ripen. So that would be another clue of a lightweight tomato.

Second let’s consider measuring **DIAMETERS** using **CALIPERS**

D1 = Length = The longest radius possible measured across the equator, perpendicular to the poles.

D2 = Width = Radius taken 90° from D1, also across the equator.

D3 = Height = Radius taken the blossom end to adjacent to the stem, at right angles to D1 and D2.

Using diameters, measured in cm, and a DAF of 0.95, the formula for Weight grams becomes:

**W=0.49742*D1*D2*D3** (weight in grams)

So for the Big Zac (4.20 Diaz 2011) B-001 pictured at the top, my latest measurements are: D1=16.13, D2=14.48, D3=8.16. This calculates to 948 g, or 2.090 lb.

This fruit is now at 37 days since fruit set. It’s growth stopped rather abruptly at 34 days – right on schedule for tomatoes grown during the heat of Summer. As mentioned elsewhere, 40 days, give or take a couple of days, is what it takes during the summer, at least where I live, for a giant tomato to go from fruit set to time to pick.

I wish I knew a way to extend the growth phase! Between day 16 and 26, this tomato averaged a 10.3% weight gain per day. Another 10 days at this rate would have put this one into the 5-lb. range.

So in the next couple of days, I’ll pick this tomato, take final meaurements using a tape measure and see how close this prediction is. I’m usually a bit disappointed, but this tomato is relatively well behaved, geometrically speaking.

If you take measurements in English units (inches), measure all 3 perpendicular circumferences (CC’s), convert these to decimal inches (i.e. 19-3/8 inches = 19.375 inches) and use this formula, where weight estimate ( W ) is in pounds:

**W=0.00056*CC1*CC2*CC3** (weight in pounds)

This uses a DAF of 0.92, so should work reasonably well regardless of stage of ripeness.

For more details about the development of this technique for estimating tomato weights, visit:

Perry’s Greenhouse Tomato Weight Estimation Thread