Goliath Bird Eater Tarantula

The first spider shape an Arachnomancer gets will be a small, medium, and large monstrous spider. Let's just talk about large size for the sake of argument.

According to a National Geographic regarding tarantulas:

Fast Facts

Type: Bug
Diet: Carnivore
Average life span in the wild: Up to 30 years
Size: 4.75 in (12 cm) long; leg span, up to 11 in (28 cm)Weight:1 to 3 oz (28 to 85 g)

According to Wikipedia:

Some of the largest species of tarantula may weigh over 85 grams (3 oz); the largest of all, the goliath birdeater (Theraphosa blondi) from Venezuela and Brazil, has been reported to attain a weight of 150 grams (5.3 oz) and a leg-span of up to 30 centimetres (12 in)

According to Big and Little Creatures in Combat (SRD):

Size Category: Large
Size Modifier: -1
Grapple Modifier: +4
Hide Modifier: -4
Height or Length: 8 ft. - 16 ft.
Weight: 500 lb. - 2 tons
Space: 10 ft.
Natural Reach (Tall): 10 ft.
Natural Reach (Long): 5 ft.

Let's just use the space it takes up; in this case it is 10 ft. So using the statistics for a biggest tarantula, it is 5.3 ounces at 12 inches. Multiplying those values by 10 would leave me at 53 ounces for a 10 ft. long spider. 53 ounces = 3.3 lbs.

Am I doing that math correctly? I am not a biologist, but I think a 10 ft. spider should weigh more than a 6 pack of beer.

Also, I do not believe that a 10 ft. spider should weigh 500 lb - 2 tons.

What exactly is a reasonable weight for a large size monstrous spider? The reason it is important is due to possibly pouncing on people from high heights.

  • 1
    \$\begingroup\$ Also note, I think that 10 inch is measuring from the front foot to the back foot, the actual body is half that size. I would say, if you want a spider that is comparable to a 10ft creature, you probably want its total length to be 13-14ft (which would give you something like a ~7ft body). \$\endgroup\$
    – Jonathon
    Aug 26, 2015 at 1:21

1 Answer 1


There's a factor you missed out on: the square-cube law. As Wikipedia describes it:

When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier.

Let's consider the implications of this for a moment with a cube of water: it will be one centimeter along each side, and will weigh one gram.

By your math, if you want a cube of water that's ten times bigger, you also just times the weight by 10, for a final weight of 10 grams. But that's just ten times the water, and I think we'll agree there's a big difference between ten spiders and a ten times taller spider. What you probably mean by 'ten times bigger' is a cube that's ten times bigger along each dimension. That gives us a 10 × 10 × 10 cm cube of water.

We'll see what the square-cube law has to say about increasing each dimension by 10, using that 10 as our size multiplier:

  • The surface of one side goes from 1 square centimetre to 100 square centimetres. That means the cube's total surface area goes from 6cm² to 600cm². This surface area got increased by the square of the multiplier, 100.
  • The volume of the cube goes from 1cm³ to 1,000cm³ (10 × 10 × 10cm). This volume got increased by the cube of the multiplier, 1000.
  • Since the weight of water is 1 gram per cubic centimetre, this now weighs 1,000g, or a kilogram.

Applying this to a spider

Let's assume the spider's anatomy remains identical as it gets proportionately bigger, and that an increase in volume leads to an exactly proportional increase in weight. (In reality, an enormous spider would need to have much sturdier anatomy, better breathing systems, and other stuff in order to be able to avoid suffocating and its own exoskeleton caving in on it, but they might not know that. Let's assume imaginary radiation magic is helping here.)

Original: 5.3 ounces at 12 inches.

Multiplication factor: 10, for going from 1 foot across to 10 feet across.

New weight: 5.3 × 10³ = 5.3 × 1,000 = 5,300 ounces = 331 pounds.

This is about the weight of a very young elephant (they're ~200 pounds when born). The large entry suggests creatures this big should weigh at least 500 pounds, so either this is spider is a near-the-mark outlier, or large spiders have actually developed much sturdier, denser, stronger anatomy for their size and thus weigh a lot more. Imagining an enormous spider with more elephantine body/leg proportions is awesome and terrifying for me.

Enjoy your new spider!

  • 2
    \$\begingroup\$ ~331lbs is just shy of the 500lb mark, but it makes sense add most spiders and insects are probably below 'average density' (for a moment I considered applying BMI to spiders) so that checks out. \$\endgroup\$ Feb 9, 2015 at 8:33
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    \$\begingroup\$ @Pureferret if anything that below average density might be a contributor to why if you enlarge them like this their body will crush itself under its own weight. That ~500lb mark is probably the mark for a spider properly beefed up to handle its own size. \$\endgroup\$ Feb 9, 2015 at 8:35
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    \$\begingroup\$ The have been big insects in the past: en.m.wikipedia.org/wiki/Largest_prehistoric_animals I don't know if size was down just increased oxygen in the atmosphere or also some physiological factor. \$\endgroup\$ Feb 9, 2015 at 8:46
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    \$\begingroup\$ @Pureferret The largest arthropod even then was 8.5 ft., quite a bit short of this 10 ft., and that was a millipede rather than a spider (which seems likely a sturdier construction) and relied on the substantially greater partial pressure of oxygen in that era (the lack of which sharply limits terrestrial arthropod size today). So without magic, a spider doesn't work simply scaled up. \$\endgroup\$
    – KRyan
    Aug 26, 2015 at 4:25
  • 1
    \$\begingroup\$ @kryan totally agree with you, I think the original creature I was thinking of was a prehistoric trilobite. \$\endgroup\$ Aug 26, 2015 at 9:21

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