Conversion of Unit of Measure Not Possible (M8 845)

A supplier invoice lands in your matching queue and SAP will not post it. The message reads Conversion of unit of measure & to & for mat. & & not possible. You open the material: the price agrees with the contract, the delivered quantity agrees with the goods receipt, the purchase order lines up. And still the system cannot reconcile the unit of measure on the invoice with the material’s base unit of measure, so it holds the document. A buyer is already drafting a query to the vendor who did nothing wrong, and accounts payable is staring at a line that seems to hang for no reason. This message is almost always misread — as a price or quantity discrepancy, as the vendor’s mistake, or as the well-known production message with nearly the same wording. None of those is what you have. What is missing is a conversion factor in master data that ties the invoiced unit to the base unit. This guide shows which message you are actually looking at, why the informal name for it misleads, and how to close the gap at the source instead of clearing every affected invoice by hand.

Where the conversion failure actually hides

The phrase “unit of measure cannot be converted” appears in several modules, under different numbers and different root causes. The table below separates them.

In invoice verification exactly one of these applies: M8 845. The others describe the same physical defect — a missing or wrong conversion factor — but at a different point in the system: MM 101 and MM 328 when the material master is accessed, 06 232 at the purchasing info record. The last row is the dangerous one, because its short text sounds almost identical and is often used as the name for the whole problem — yet RU 077 belongs to the production confirmations and has nothing to do with the incoming invoice. Follow the wording instead of the message number and you hunt for the cause in the wrong module.

Before and after: chasing the symptom vs. maintaining the conversion

Maintain the conversion at the source — do not just change the unit on the invoice

1. 1
   Read the message number and class, not just the wording

   Confirm first that you really have M8 845. The nearly identical sentence from RU 077 comes from the production confirmations and leads to a completely different investigation — one about confirmed manufactured quantities, not a supplier invoice. In invoice verification, M8 845 is the message that applies, and the class in front of the number tells you which module to look in at all.

2. 2
   Check the alternative unit of measure and its factors in the material master

   Open the material in MM02 (Industry) or MM42 (Retail) and confirm that every invoiceable unit carries a conversion factor to the base unit of measure. SAP names this step in the remedy to MM 101: enter the conversion factors to the alternative unit of measure in the material master record. A unit and dimension converter confirms the factor in seconds.

3. 3
   Check table T006

   M8 845 names the remedy itself: check the entries in table T006, which holds the units of measure, for correctness and completeness. A unit that is undefined or wrongly defined there cannot be converted anywhere.

4. 4
   Include historical and period units

   M8 845 can involve units that no longer match the material’s current base unit of measure. Reconcile any older unit against the current base unit so no residual unit is left without a path to the base.

5. 5
   For 06 232, reconcile the info record

   If purchasing is involved, compare the base-unit-to-order-unit conversion in the info record’s general data with the order-unit-to-order price unit conversion in its organisation data. Both must form one consistent chain, or SAP divides the invoice price against the wrong quantity.

Build a conversion check into your invoice matching

• The message number and class are identified — M8 845 (invoice verification) is told apart from RU 077 (production confirmation) before anyone starts searching
• Every invoiceable unit carries a conversion factor to the base unit in the material master (MM02 Industry / MM42 Retail)
• The material’s alternative units of measure are checked for correctness and completeness
• The units involved are defined completely and correctly in table T006
• Any older unit is reconciled against the current base unit of measure so none is left without a conversion path
• In the purchasing info record, the conversions in general and organisation data agree with each other (no 06 232)
• The maintained factor is written back to the material so every later invoice inherits it instead of being corrected one at a time

Why identity resolution is the real fix

A unit that cannot be converted is an identity problem underneath: the system cannot line up the unit a supplier invoices in with the unit you stock, order, and pay in. Patch one invoice by hand and the next unit fails at the same gap, because the factor was never written back to the trusted source. The durable fix is to resolve each material to one record whose units of measure and conversion factors are known and consistent — which is exactly what invoice reconciliation against trusted catalog data does. It is the same mechanism the flagship guide The Error Says Price. The Cause Is Units. describes across every ERP: get unit and identity right once, and the downstream invoice stops lying.

Related

FAQ

REVIEW-NOTES

• Core disambiguation confirmed. RU 077 (Alternative qty unit & cannot be converted into base unit &) is correctly attributed to message class RU / production confirmations; M8 845 (Conversion of unit of measure & to & for mat. & & not possible) is the invoice-verification message (class M8). This distinction is built out deliberately as core page content (table, callout, FAQ Q2).
• Every number traces to the brief appendix. M8 845, MM 101, MM 328, 06 232 and RU 077 with their short texts and remedies; table T006 (units of measure) and the material-master alternative units / conversion factors; MM02 (Industry) / MM42 (Retail); the 06 232 info-record general-vs-organisation-data cause. SAP ampersand placeholders (& to & for mat. & &, & cannot be converted into base unit &) are preserved and short texts are set as code.
• M8 845 long text / “historical and period units” step. Step 4 references older/period units generically; the verified English appendix for M8 845 carries only the short text and the T006 remedy (the period-set long text is documented in the German appendix only). Phrasing kept non-specific to avoid asserting an un-verified English long text — spot-check against SE91 / the English long text before publication if a sharper claim is wanted.
• Hub / sibling wiring. Inline links point only to pages that exist today: tools/unit-dimension-converter, glossary/order-price-unit, glossary/unit-of-measure, the flagship guides/error-says-price-cause-is-units, and the external by-use-case/invoice-reconciliation. The German twin guides/sap-mengeneinheit-nicht-umrechenbar is Card-linked only and labelled “Guide (German)”, per instructions (not inlined). When the sibling ERP guides ship, consider adding translations: { de: "sap-mengeneinheit-nicht-umrechenbar" } here and wiring relatedSlugs for the audit-checked mirror.
• DE/IT CTA localisation remains the open shared-code decision in SCHEMA-NOTES §4; it does not affect this English page, whose book-demo CTA renders the brief’s heading and button verbatim.