Tarantula Sling Sex Odds Calculator (Free Tool)
Updated: July 27, 2026
Buying unsexed tarantula slings is basically a lottery. This calculator turns "how many slings should I buy to get a female" into real numbers, based on how many you buy and the odds that any single sling turns out female.
The default assumption is a simple 50/50 male-to-female split, which is the standard estimate keepers use for random unsexed slings. You can adjust that percentage if you believe a particular species, line, or seller's slings skew more male- or female-heavy.
What this calculator is for
- Estimating your odds of ending up with at least one (or more) female from a batch of unsexed slings.
- Working out how many slings you'd realistically need to buy for a given confidence level, like "95% sure I get a female."
- Sanity-checking a bulk or "grab bag" sling purchase before you commit.
This is a statistics tool, not a guarantee. It assumes each sling's sex is an independent coin flip at whatever percentage you set. Real-world eggsacs and lines are not always perfectly independent or perfectly 50/50. See the FAQs below for more on that.
Your odds
Show full probability breakdown
"Exactly" is the odds of landing on that exact number of females. "At least" is the odds of getting that many or more, which is usually the number you actually care about.
How many slings would I need to buy?
Flip the question around: instead of picking a number of slings and checking the odds, set a confidence level you're comfortable with and see the minimum number of slings that gets you there.
FAQs
Is the male/female ratio for tarantula slings really 50/50?
Nobody knows the exact ratio for a given eggsac ahead of time. 50/50 is just the standard working assumption keepers use for random, unsexed slings, so that's the default here. Some keepers and breeders think certain species or lines run more male-heavy or female-heavy in practice. If you have a reason to believe that about the batch you're buying from, adjust the percentage field above.
If I buy more slings, am I guaranteed a female?
No. Buying more slings raises your odds, but it doesn't guarantee anything, same as flipping a coin more times doesn't guarantee heads. The reverse calculator above shows how many slings it takes to reach a confidence level you're comfortable with, like 95% or 99%. Even a 99% chance still leaves room to get unlucky.
Does this account for slings from the same eggsac not being truly independent?
No. This calculator treats every sling as an independent coin flip at whatever percentage you set, which is the standard simplification for this kind of math. In practice, sex ratios within a single eggsac can sometimes cluster more than pure independent odds would predict. Treat the numbers here as a solid planning estimate, not an exact guarantee.
Where can I buy tarantula slings?
Use the main Tarantula List comparison tool to see current listings across vendors, including which ones sell slings sexed versus unsexed.
Is there a way to guarantee I get a female?
Buying a sling that's already been sexed removes the guesswork. Molt confirmation, examining a shed exoskeleton for the physical markers of sex, is the gold standard for sexing a tarantula and is far more reliable than guessing from looks alone. It costs more than buying unsexed, but you know exactly what you're getting. Once you've built out a collection, you can track sexes, molts, and enclosures with Keeper by Tarantula List, and plan shelf space with the Enclosure Shelf Planner.