Two different numbers get called "what it's worth" -- the settlement and the salvage value. They're not the same.
On this page
When a car is totalled there are two separate values in play, and mixing them up is the source of most disappointment. The first is actual cash value: what the car was worth immediately before the accident. That's what the insurer settles on, less your deductible. The second is salvage value: what the damaged car is worth as it now stands. That's what the insurer deducts if you keep it, and roughly what you'd get selling it as-is. Asking "what is my totalled car worth" usually means the second one, but the answer you find online is usually about the first.
Four things, roughly in order of importance. What's damaged. Cosmetic damage on a mechanically sound car retains far more value than anything structural, and structural damage is the sharpest drop of all. What survived. An intact drivetrain, a clean interior, and undamaged panels on the opposite side are what parts buyers pay for. Demand for the model. Common cars have deep parts markets; unusual ones can be worth more or almost nothing depending on who's looking. And whether it runs and drives, which widens the pool of buyers considerably.
Valuation guides price clean, undamaged cars. There's no book value for "2015 sedan, front-end collision, runs, frame straight" because that description covers cars worth wildly different amounts depending on the quality of the assessment. Rules of thumb -- a percentage of clean-title value -- are a starting point at best. The spread around them is enormous. The only reliable method is comparison: find damaged cars of the same model and similar damage, and see what they're actually listed and sold for. That's real market data rather than a formula.
Start with the insurer's figure. If you're retaining the car, the salvage deduction they quote is a professional estimate of exactly this value -- and it's free. Then check it against the market. Browse comparable salvage and rebuilt listings for your make and model, filtered to a similar title status, and look at the range rather than any single car. If the car is worth serious money, get a quote from more than one buyer -- a salvage yard, a rebuilder, and the open market will not agree, sometimes by a wide margin.
A salvage yard buys to resell at a profit, so their offer is necessarily below what the car is worth to an end buyer. It's fast and certain, which has real value if that's what you need. Selling directly to a rebuilder or a parts buyer generally realises more, because you're capturing the margin the yard would have taken. It takes longer and needs a decent listing -- honest photographs of the damage, a specific description, and any repair or inspection paperwork. If you have the patience, the difference is often substantial.
It depends which number you mean. The settlement is the car's value before the accident, less your deductible. The salvage value -- what the damaged car is worth now -- depends on what's damaged, what survived, and demand for the model. Comparing against real listings for similar damaged cars is the most reliable guide.