OPM Backsolve vs PWERM: Key Differences Explained

The table compares the two methods on what each is, who uses it, its inputs, legal and tax treatment, the main risk, how it relates to liquidity and a typical example. Both methods answer the same question. They answer it from different starting points.

OPM Backsolve

The OPM backsolve begins with a fact. Investors just bought preferred stock at a known price. The appraiser builds an option model of the whole cap table. Each class is treated as a call option on the company’s value, a right that pays only above a certain level. The appraiser then searches for the total equity value at which the model gives that exact preferred price.

Once the total is found, the model splits it among the classes. Common stock sits behind the liquidation preference, so it gets less per share. A discount for lack of marketability then lowers it further, because common shares cannot be sold easily.

PWERM

PWERM begins with a forecast. The appraiser lists the ways the company could end: an IPO, a sale or a wind-down. For each one, it estimates the exit value and the payout to each class. It weights the payouts by probability and discounts them to today.

An illustrative case, with invented numbers. A company expects one of three outcomes in two years:

• IPO, 50% chanceCommon stock worth $12 a share
• Sale, 35% chanceCommon stock worth $6 a share
• Wind-down, 15% chanceCommon stock worth $0
• ResultWeighted value $8.10 → $5.18 after a 25% yearly discount rate → $4.15 after a 20% marketability discount

A hybrid method is a PWERM in which one scenario is valued with an OPM. Inozyme used one in March 2020. It valued an IPO scenario from recently listed peers and an unspecified exit with a backsolve.

Neither method favors one side by design. The test is whether the method fits the company’s facts. A backsolve fits when a fresh round exists and no exit is planned. PWERM fits when bankers are hired and an IPO window can be modeled.

For employees, the method matters through the exercise price. A lower common value means a lower price on new options. A value that is too low invites tax trouble, so appraisers must defend every input. For investors, the method explains the gap between their preferred price and the common value.

Inozyme shows the gap. It sold Series A preferred at $1.00 a share on November 8, 2018. Its appraisal as of November 30, 2018 valued the common stock at $0.25. As an IPO came into view, it moved to a hybrid. The common value rose from $0.27 in May 2019 to $0.37 in March 2020.

The tax rules are the same for both methods. Under Section 409A, private company stock must be valued by the reasonable application of a reasonable method. The regulations list factors such as the company’s assets, its future cash flows and recent arm’s-length sales of its stock. They do not name the OPM or PWERM.

An independent appraisal dated no more than 12 months before a grant is presumed reasonable. The IRS can overturn it only by showing the method, or how it was applied, was grossly unreasonable. This appraisal is the 409A valuation. To keep that presumption, a company refreshes its appraisal at least every 12 months.

If options are priced below fair market value, they can fall under Section 409A. The holder then owes income tax early, interest and an extra 20% tax. The method choice does not change these rules. It changes the number the rules are applied to.

OPM Backsolve

For

It starts from a real price paid by outside investors. It needs no forecast of exit values or their odds. It handles complex share classes in one model.

Against

The round may include terms the common stock lacks. Volatility and time-to-exit guesses can move the result a lot. It ignores a known exit plan.

Verdict: the backsolve is the natural choice soon after a priced round, when no exit is in sight. Its strength is the anchor in a real transaction. Its weakness is that the anchor ages, and a stale round gives a stale value.

PWERM

For

It shows how each share class fares in each exit. It reflects a planned IPO or sale directly. Its logic is easy to follow.

Against

Every probability and exit value is a judgment. Far from an exit, the scenarios are hard to support. Small changes in the odds can swing the result.

Verdict: PWERM fits a company preparing an IPO or a sale. The closer the exit, the easier the scenarios are to defend. Many appraisers use a hybrid at that stage, as Inozyme did, to keep a backsolve for the case where the IPO does not happen.

If you receive options, ask which method the latest appraisal used and its date. A value more than 12 months old loses its presumption. A new round or a planned IPO can also make it stale. For how the common value relates to the round price, see 409A valuation vs preferred price.

If you buy shares in a secondary sale, remember that a 409A value is a tax figure. Your price is set by the deal. For the wider toolkit, see private company valuation and common stock vs preferred stock.

Common Mistakes

• Reading the 409A value as a market priceIt values common stock for tax and accounting. Buyers in a secondary sale may pay more or less.
• Expecting common to equal the preferred pricePreferred stock is paid first in an exit. Inozyme’s common was valued at a quarter of its preferred price.
• Treating one method as the IRS’s choiceThe rules require a reasonable method. They do not name either one.
• Ignoring the dateA value is reasonable for 12 months at most. A new round or IPO plan can age it sooner.

OPM backsolve

A method that finds the total company value implied by a recent round price, then splits it among share classes with an option model.

PWERM

The probability-weighted expected return method. It values share classes across possible exits, weighted by probability and discounted to today.

Hybrid method

A PWERM in which one scenario is valued with an option pricing method.

Option pricing method (OPM)

A model that treats each share class as a call option on the whole company’s value.

Call option

A right that has value only when the underlying value rises above a set level.

Liquidation preference

The right of preferred holders to be paid back before common holders in a sale or wind-down.

Discount for lack of marketability

A reduction for shares that cannot be sold easily, applied after the method sets a value.

409A valuation

An appraisal of private company common stock used to price stock options under US tax rules.

Arm’s-length transaction

A sale between unrelated parties, each acting in its own interest.

Figures were last checked on November 6, 2026. The rules are United States federal tax rules. Each appraisal follows its own assumptions, and a company’s figures are the ones in its own filings.