About FormulaEngineer

FormulaEngineer is a .NET library that reads, writes, and accurately evaluates Excel formulas — all without requiring Excel to be installed.

Most .NET Excel libraries can open and create .xlsx files. Very few can evaluate the formulas inside them correctly . FormulaEngineer is built around that gap: a from-scratch formula engine that matches Excel's output to last-bit floating-point precision across the lookup, financial, dynamic-array, and reference function families.

It's built for production workloads that need to process spreadsheets at scale, on any platform, with strict correctness guarantees on formula evaluation. Pure managed code, .NET 6 and later, Windows, Linux, and macOS.

Most .NET Excel libraries either skip formula evaluation entirely (delegating to Excel COM interop, which doesn't work on Linux or servers) or implement a small subset incorrectly. FormulaEngineer takes a different approach: a from-scratch engine with strict correctness as the design goal.

FormulaEngineer has been verified against hundreds of formulas from production-style workbooks covering real-world financial, lookup, aggregate, and reference patterns. Where Excel's cached values exist in the source workbooks, results match bit-for-bit.

Coverage spans 315+ Excel functions across all major families — lookup, financial, statistical, date/time, text, math, information, and database aggregates.

Function families verified

Lookup: LOOKUP, VLOOKUP, HLOOKUP, XLOOKUP, INDEX, MATCH — including wildcards, case-sensitive matching, reverse search, multi-criteria, and structured table references.

Dynamic arrays: FILTER, SEQUENCE, TRANSPOSE, spill semantics, array-to-array concatenation, boolean-to-numeric coercion.

Financial: PMT, IPMT, PPMT, IRR, XIRR, MIRR, NPV, PV — verified to last-ULP precision against Excel's cached values across all compounding frequencies.

Date and time: DATE, EDATE, EOMONTH, YEAR, MOD, TODAY, NOW — with correct leap-year handling and end-of-month clamping.

Aggregate: SUM, AVERAGE, COUNT, COUNTA, COUNTIF, COUNTIFS, SUMIF, SUMIFS, AVERAGEIF, AVERAGEIFS, SUMPRODUCT, AGGREGATE.

Logical and info: IF, AND, OR, NOT, XOR, IFERROR, IFNA, ISBLANK, ISERR, ISNA, ISEVEN, ISODD, TYPE, ERROR.TYPE, CELL.

Text: CONCAT, TEXTJOIN, TEXT, LEFT, MID, RIGHT, LEN, SEARCH, FIND, SUBSTITUTE, EXACT, VALUE.

Patterns that break most engines

Patterns verified that typically fail in non-Microsoft formula engines: INDIRECT fed an array of sheet names with COUNTIF broadcasting across resolved ranges; header-driven nested XLOOKUP that returns an entire column dynamically; FILTER + SEQUENCE for Nth-occurrence lookup; the CHOOSE-virtual-table case-sensitive VLOOKUP trick; matrix lookups with two-row composite headers; distinct-values extraction with array-criterion COUNTIF.

Cross-platform. Pure managed .NET code targeting .NET 6 and later. No platform-specific bindings, no Excel runtime, no COM interop. Works identically on Windows, Linux, and macOS.

Streaming architecture. The reader and writer are both forward-only and single-enumeration. Memory stays flat regardless of file size — process gigabyte-scale workbooks without OOM concerns.

IEEE 754 compatible. Floating-point arithmetic matches Excel exactly, including canonical edge cases (e.g. 1.9 - 1.7 = 0.19999999999999996 — same artifact in both engines).

Honest about correctness. When a regression is found, it's patched, re-verified against the original failing workbook, and re-tested across the existing verification corpus to confirm no other formulas regressed.

FormulaEngineer is built and maintained by a .NET developer focused on spreadsheet internals. The product scope is narrow on purpose: one library, one set of capabilities, done well. Support emails reach the team that built the product.