Education
OPM vs. PWERM: How a 409A Allocates Your Company's Value Into a Common Stock Strike Price
Your total valuation and your team's strike price are two ends of one 409A calculation. How OPM, PWERM, the backsolve, and DLOM set your common stock FMV.
By 409.AI Team - 2026-07-27
Say you just closed a $120M post-money Series C. Then your 409A comes back and sets the common stock strike price at $2.87. The two numbers feel like they belong to different companies. They don't. They're two ends of the same calculation, and the part that connects them is the step most founders never see: allocation.
Valuing a startup is really two jobs stacked on top of each other. First, an appraiser estimates the total value of your equity. Second, they split that value across everything sitting in your cap table: seed preferred, the priced rounds, participating stock, options, and plain common. Employees get common, so the strike price your team pays is whatever falls out of that second step. Get the allocation wrong and the whole 409A is wrong, no matter how careful the first step was.
This piece is about that second step, and the three methods appraisers use to run it: the Option Pricing Model, the Probability-Weighted Expected Return Method, and the hybrid that borrows from both.
Valuing the company is only half the work
If you want a refresher on the first half, we've written separately on [how a 409A valuation is actually calculated](https://www.409.ai/articles/how-are-409a-valuations-calculated) and on the three lenses an appraiser uses to size total value: the [market approach](https://www.409.ai/articles/market-approach-409a-valuation), the [income approach](https://www.409.ai/articles/income-approach-409a-valuation), and the [asset-based approach](https://www.409.ai/articles/asset-based-approach-409a-valuation). Those get you to one number, the total equity value.
That number is not your strike price. Preferred shareholders hold rights that common holders don't: liquidation preferences that pay them first, sometimes participation on top, sometimes a dividend. Those rights have value, and that value has to come out of the pie before common gets its slice. Allocation is how an appraiser decides who gets what, and it's the reason your team's strike price sits well below the price your investors just paid. We dig into that gap on its own in [why your 409A comes in lower than your post-money valuation](https://www.409.ai/articles/why-is-your-409a-valuation-lower-than-post-money-valuation).
The standard reference for how to do this is the AICPA's accounting and valuation guide, *Valuation of Privately-Held-Company Equity Securities Issued as Compensation*, known around the industry as the "cheap stock" guide. It lays out the allocation methods below and when each fits.
The Option Pricing Model: every share class is a call option
The Option Pricing Model, or OPM, starts from a clever idea. Common stock only pays off once the company clears every senior claim above it. That makes common behave like a call option: it's worth something only above a certain threshold, and nothing below it.
The OPM turns your cap table into a series of these thresholds, called breakpoints, where the economics change as company value rises. Below a preferred stack's liquidation preference, all the money goes to preferred. Above it, common and preferred start sharing. Each slice between breakpoints gets priced as its own option using Black-Scholes, and the values are added back up to give each share class its worth.
A simplified version: imagine total equity value of $100M and a single preferred class with a $30M liquidation preference. Below $30M in an exit, preferred takes everything. Above it, common participates. Common is essentially a call option struck at that $30M breakpoint, priced with Black-Scholes.
Two inputs drive the result more than any others. The first is volatility, usually pulled from comparable public companies in your sector. The second is expected time to a liquidity event, meaning how many years until an IPO or sale. Both push in the same direction: more volatility and a longer runway to exit give the common "option" more time and more room to land in the money, which lifts the common value. A company two years from a likely sale and a company that's seven years out can carry identical total valuations and still land at very different strike prices.
The backsolve: pricing off the round you just raised
The OPM has a favorite trick, and it's the reason most venture-backed startups get a clean 409A. It's called the backsolve.
Instead of guessing total equity value from scratch, the appraiser works backward from a number you already know: what investors just paid in your most recent priced round. They solve for the total equity value that, once run through the OPM allocation, hands your newest preferred series exactly the per-share price those investors paid. If your Series C priced at $9.40 a share, the model tunes total value until the OPM spits out $9.40 for Series C. Everything else, including common, falls out of the same run.
The backsolve is popular because it's grounded in a real, arm's-length transaction rather than a forecast. It works best when the round is recent, priced (not a [SAFE](https://www.409.ai/articles/how-safes-affect-your-409a-valuation) or note), and led by outside investors. Push it too far past its freshness date and auditors start asking questions, which is exactly why 409As expire and get refreshed.
PWERM: when the exit comes into focus
The OPM is built for uncertainty. It's happy to model a company that could go a dozen directions over an unknown number of years. But some companies aren't that uncertain anymore. A pre-IPO business with a filing range, or a startup deep in sale talks, has a much clearer set of futures.
That's where the Probability-Weighted Expected Return Method, or PWERM, comes in. Rather than treating value as one continuous option, PWERM lays out specific exit scenarios: an IPO at this valuation, a strategic acquisition at that one, a down-side sale, maybe a dissolution. The appraiser assigns each scenario a probability and an exit value, walks the proceeds down the cap table waterfall in each one to see what common actually collects, then weights the results.
PWERM shines when you can genuinely picture the exits and roughly time them. It struggles when you can't, because the whole method rests on the probabilities and payouts you feed it, and early-stage guesses about a five-year exit aren't worth much. That's why you rarely see pure PWERM on a Series A.
The hybrid: probability-weighting both worlds
Most later-stage 409As don't pick a side. They use the hybrid method, which runs PWERM and OPM together.
The appraiser carves out the near-term, modelable exits, an imminent IPO or a live acquisition, and values common in those scenarios with a PWERM waterfall. Then everything else, the "stay private and keep growing" residual, gets bundled into a single OPM scenario that captures the wide range of longer-term outcomes. The final common value is the probability-weighted blend of the two.
The hybrid has become the default for companies that can see an exit on the horizon but haven't locked it in. It's honest about what you know (the near-term paths) and what you don't (everything after), which is usually the real situation.
Why common lands below preferred: the waterfall and DLOM
Two forces pull the common strike price down. The first is the waterfall itself. Preferred gets paid first, so in a lot of outcomes, especially modest exits, common receives little or nothing. Allocation prices that reality in.
The second is the discount for lack of marketability, or DLOM. Your investors can't easily sell their shares, but at least they hold rights and, increasingly, access to secondary liquidity. A rank-and-file employee holding common has neither. Because that stock can't be freely sold, appraisers apply a DLOM after allocation, often somewhere in the 20% to 35% range depending on how far off liquidity looks and how volatile the sector is. A longer time to exit generally means a larger discount. Stack the waterfall and the DLOM together and you get the spread between the $9.40 your investors paid and the $2.87 your team pays.
If you want to see where these show up in a real report, our [section-by-section walkthrough of a 409A report](https://www.409.ai/articles/decoding-a-409a-valuation-report-walkthrough) points to the exact pages.
What the AICPA's 2025 draft signals
There's a live reason this topic is worth your attention right now. In December 2025, the AICPA released a working draft updating the cheap stock guide, with a comment period running into mid-2026. Two of the reworked chapters speak directly to allocation: guidance on splitting value among share classes in complex cap tables, and, notably, how appraisers should treat secondary market transactions.
That second one matters because of where the market is heading. Companies are staying private far longer than they used to, and a maturing secondary market has stepped in to give employees and early investors liquidity that an IPO used to provide. When your own stock trades in a tender offer, that price becomes evidence an appraiser has to weigh, and it can move your common FMV in ways a clean backsolve wouldn't. We cover the mechanics of that in [what tender offers and secondary sales do to your 409A](https://www.409.ai/articles/tender-offers-secondary-sales-409a-valuation). The AICPA sharpening its guidance here is a signal that scrutiny of secondary-informed valuations is rising, not fading.
Which method fits your company
There's no universally "right" method, only the right fit for your stage and structure. Early, venture-backed, and years from any exit? An OPM backsolve off your latest round is usually the cleanest, most defensible answer. Later-stage with a plausible IPO or sale in view? Expect a hybrid, and a pure PWERM only when the exit is close and the scenarios are real. A quiet company with no recent financing and no exit in sight is the hardest case, and the one where an experienced appraiser earns their fee by choosing inputs that will survive an audit.
The practical takeaway: when you read your next 409A, don't stop at the total valuation on the cover. Turn to the allocation section. The method chosen there, and the volatility, time-to-liquidity, and DLOM assumptions behind it, are what actually set the price your employees pay to own a piece of what they're building. If those assumptions don't match the company you recognize, that's the conversation to have with your appraiser, before the grants go out, not after.