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This article explains the real-world differences between legacy CSE array formulas and modern dynamic array formulas in Excel, and provides a practical migration strategy so that advanced users can redesign workbooks for better performance, maintainability, and compatibility.
1. Conceptual overview: legacy arrays vs dynamic arrays
Excel supports two fundamentally different array calculation engines in modern versions: legacy CSE array formulas and new dynamic array formulas.
1.1 Legacy CSE array formulas
Legacy array formulas are formulas that operate on multiple values at once, entered with the key combination Ctrl + Shift + Enter (CSE). Excel internally flags them as array formulas and wraps them in braces in the formula bar.
{=SUM(A1:A10*B1:B10)}Key characteristics of legacy CSE formulas include the following.
- The formula is usually entered in a single cell, but can also be entered over a fixed multi-cell range as a block array.
- The formula result range is static. If the underlying data range grows or shrinks, the array output range does not automatically resize.
- Editing any cell in a multi-cell legacy array requires selecting the entire array range first.
- Legacy arrays are supported in older Excel versions (pre-dynamic arrays), which makes them a compatibility tool.
1.2 Dynamic array formulas
Dynamic array formulas are formulas that return a variable-sized array and spill into adjacent cells automatically. They rely on the newer calculation engine introduced in Microsoft 365 and recent perpetual versions.
=FILTER(A2:B100, A2:A100="East") =UNIQUE(A2:A100) =SEQUENCE(10,1,1,1)Core concepts behind dynamic arrays include the following.
- One formula can populate many cells by spilling results.
- The spill range automatically grows or shrinks when the underlying data size changes.
- Many functions now natively return arrays without CSE, for example FILTER, SORT, UNIQUE, SEQUENCE, XLOOKUP, and others.
- Dynamic behavior is visible via the
#SPILL!error when the spill range is blocked.
2. Feature comparison: legacy vs dynamic array formulas
The table below summarizes the most important behavioral differences between legacy and dynamic arrays in Excel.
| Aspect | Legacy CSE array formulas | Dynamic array formulas |
|---|---|---|
| Entry method | Requires Ctrl + Shift + Enter | Enter with standard Enter key |
| Result range | Fixed single cell or fixed block range | Spill range automatically resizes |
| Editing | Whole array range must be edited at once | Edit the top-left cell only |
| Error behavior | Standard error values (e.g., #VALUE!) | Dynamic errors such as #SPILL! and #CALC! |
| Supported versions | All desktop versions; necessary for older Excel | Microsoft 365 and newer dynamic array versions |
| Performance characteristics | Can be expensive in large ranges; recalculates entire array | Engine optimized for array operations and spill logic |
| Maintainability | Hard to audit; CSE behavior is hidden | Formula visible once; spill range is intuitive |
| Design pattern | Array logic encapsulated inside aggregation functions | Use of intermediate spilled arrays composed with other functions |
Note : In dynamic array-enabled Excel, many older array formulas will still work without pressing Ctrl + Shift + Enter, but their behavior may change if rewritten using spill logic. Treat any migration as a design change, not a one-to-one syntax swap.
3. Practical examples: rewriting legacy array formulas as dynamic arrays
This section shows concrete formula patterns where dynamic arrays can replace traditional CSE formulas, improving clarity and scalability.
3.1 Multi-condition SUM with arrays
A common legacy array approach to multi-condition SUM is as follows.
{=SUM((Region="East")*(Product="A")*Sales)}With dynamic arrays, you can replace this pattern using SUMIFS or FILTER depending on your requirement.
- Using
SUMIFSwith structured ranges:
=SUMIFS(Sales, Region, "East", Product, "A")- Using
FILTERplusSUMfor more complex logical expressions:
=SUM(FILTER(Sales, (Region="East")*(Product="A")))The FILTER approach supports arbitrary logical expressions using Boolean multiplication or addition, while SUMIFS is optimized for simple criteria and may be faster on very large datasets.
3.2 Return multiple matching rows (dynamic vs CSE)
Legacy Excel often used CSE formulas combined with SMALL and IF to return multiple matches.
{=IFERROR(INDEX($B$2:$B$100, SMALL(IF($A$2:$A$100=$E$2, ROW($A$2:$A$100)-ROW($A$2)+1), ROWS($G$2:G2))), "")}Modern Excel can achieve the same result with a single FILTER formula that spills.
=FILTER(B2:B100, A2:A100=E2, "No matches")The dynamic array version is shorter, easier to audit, and does not require copying the formula down.
3.3 Top N results with dynamic arrays
Legacy arrays typically combine LARGE or SMALL with IF inside CSE formulas.
{=LARGE(IF(Category="A", Sales), ROW(1:1))}To retrieve the top N values dynamically, modern Excel can use SORT and TAKE (where available) or SORT combined with INDEX.
- Top N sales for category A with dynamic arrays:
=TAKE(SORT(FILTER(Sales, Category="A"),, -1), N)If TAKE is not available, INDEX and SEQUENCE can be combined.
=INDEX( SORT(FILTER(Sales, Category="A"),, -1), SEQUENCE(N) )Note : When converting a legacy top-N CSE solution to dynamic arrays, always review how ties and duplicate values are handled. The new pattern may return more or fewer rows than the original depending on your filter and sorting logic.
4. Spill ranges, references, and new error types
Dynamic array formulas spill their results into a flexible range. Understanding spill ranges and the new referencing syntax is essential for designing robust models.
4.1 Spill range notation and references
The cell where a dynamic array formula is entered is the anchor for the spill range. Excel visually outlines the spilled area. You can refer to the entire spill range using the # operator.
=UNIQUE(A2:A100)If the formula above is in cell D2 and spills down, another formula can reference the entire spilled result with:
=COUNTA(D2#)This syntax makes it easy to pass spilled arrays into other functions without manually adjusting ranges.
4.2 The #SPILL! error
#SPILL! indicates that a dynamic array formula cannot place all its results because one or more cells in the intended spill area are not empty or are merged or otherwise incompatible.
- Clear or move the blocking content.
- Remove merged cells in the target range.
- Check for hidden shapes or overlapping ranges in complex dashboards.
Note : In design-critical worksheets, reserve space intentionally for spill ranges. Use consistent layout patterns so that new formulas will not accidentally overlap each other and cause
#SPILL! errors.4.3 The #CALC! error
#CALC! indicates a calculation problem specific to dynamic arrays; for example, an operation that implicitly requires a single value but receives a non-reducible array. Review the formula to ensure that array reductions (such as SUM, MAX, or other aggregations) are applied where necessary.
5. Performance considerations: when arrays help and when they hurt
Both legacy and dynamic array formulas can improve or hurt performance depending on how they are applied. Dynamic arrays do not automatically guarantee faster workbooks; the underlying logic still matters.
5.1 Legacy array performance characteristics
- Legacy arrays often process entire columns or large ranges with thousands of rows in a single formula.
- When used with volatile functions (e.g., OFFSET, INDIRECT, NOW, TODAY), legacy arrays can trigger frequent recalculation.
- Complex CSE formulas with nested IF and lookups are harder to optimize, because the intent is not obvious to other users.
5.2 Dynamic array performance characteristics
- Dynamic arrays can reduce the number of repeated formulas; one spilling formula replaces dozens or hundreds of copied formulas.
- Functions such as FILTER and SORT are optimized for array operations and often perform well on typical business-sized ranges.
- However, repeatedly nesting FILTER inside other dynamic functions over whole-column references can create heavy recalculation chains.
Note : For large datasets, prefer structured tables with explicit column ranges (e.g.,
Table1[Amount]) and avoid entire-column references inside array formulas unless you have specifically measured and accepted the recalculation overhead.6. Compatibility and version-strategy
When planning a migration from legacy CSE formulas to dynamic arrays, version compatibility is a central concern.
6.1 When to keep legacy array formulas
Retain CSE formulas in the following scenarios.
- Workbooks must operate reliably in older Excel versions that do not support dynamic arrays.
- The model is stable and heavily tested, with no current need for redesign or extended functionality.
- Users are trained on the existing array techniques and the cost of retraining outweighs the benefits of migration.
6.2 When to migrate to dynamic arrays
Migration to dynamic arrays is recommended when:
- All critical users run Excel versions with full dynamic array support.
- You want to simplify complex CSE formulas into clearer, composable expressions.
- The workbook frequently needs to adapt to changing dataset sizes.
- You are introducing new reporting layouts where spill ranges provide more flexibility than fixed blocks.
6.3 Mixed environments
In mixed environments where some users have dynamic arrays and others do not, you must choose a strategy.
- Maintain a compatibility branch where CSE formulas are preserved for legacy users.
- Create a modern branch using dynamic arrays for advanced users needing new capabilities.
- Document version dependencies clearly, including a dedicated worksheet that states minimum required Excel builds.
7. Design patterns for modern dynamic array modeling
Modern Excel encourages a slightly different way of thinking about models, emphasizing re-usable spilled arrays, intermediate logical steps, and cleaner aggregation.
7.1 Use helper spilled arrays instead of opaque CSE formulas
Rather than hiding complex Boolean logic inside a single CSE expression, split the logic into multiple spilled ranges.
- Example: separate the filter logic from the aggregation:
=FILTER(DataTable, (Region="East")*(Month>=StartMonth)*(Month<=EndMonth)) =SUM(INDEX(FILTER(DataTable[Sales], (Region="East")*(Month>=StartMonth)*(Month<=EndMonth)),,1))The first formula creates a reusable spilled subset of the original data. The second performs the aggregation. This pattern increases transparency and eases debugging.
7.2 Use BYROW, BYCOL, and MAP when available
In newer builds, functions like BYROW, BYCOL, and MAP let you apply custom row-wise or column-wise logic to arrays without manual CSE constructs.
=BYROW(SalesRange, LAMBDA(r, SUM(r)))Such patterns replace legacy formulas that relied on helper columns or deeply nested array calculations across rows.
7.3 Replace OFFSET/INDIRECT with structured references
Legacy array models often use OFFSET or INDIRECT to produce dynamic ranges inside CSE formulas. Modern models should favor structured tables with explicit columns and dynamic arrays for resizing.
- Legacy pattern:
{=SUM(OFFSET(A1, 0, 0, LastRow, 1)*OFFSET(B1, 0, 0, LastRow, 1))}- Dynamic table-based pattern:
=SUM(Table1[Quantity]*Table1[Price])Combined with dynamic arrays, structured references remain robust even as rows are inserted or removed.
FAQ
Do I still need legacy CSE array formulas in modern Excel?
In many cases you do not need to create new CSE formulas in modern Excel, because dynamic arrays and newer functions such as FILTER, UNIQUE, SORT, and XLOOKUP can cover the same scenarios with simpler syntax.
However, if you need strict compatibility with older Excel versions that do not support dynamic arrays, maintaining existing CSE formulas or designing new ones may still be necessary.
Can I convert a legacy array formula directly into a dynamic array formula?
There is no universal one-click conversion. Some legacy formulas will work as regular formulas without Ctrl + Shift + Enter, but fully leveraging dynamic behavior usually requires rethinking the design, including spill ranges and potentially new functions like FILTER and SORT.
The safest approach is to rebuild the logic step by step using dynamic patterns, verify results, and then retire the old CSE formula.
Why do I get #SPILL! after replacing a CSE formula?
Dynamic arrays require an empty area to spill their results. If the cells where the array wants to spill contain values, formatting, or are part of merged regions, Excel raises #SPILL!.
To fix this, clear or move the blocking content and avoid overlapping multiple spill ranges in the same layout area.
Are dynamic array formulas always faster than legacy array formulas?
Not always. Dynamic arrays reduce duplicated formulas and benefit from a newer engine, but a poorly designed dynamic formula that repeatedly filters or sorts large ranges can be slower than a well-designed legacy solution.
Performance depends on range sizes, use of volatile functions, and overall workbook design. Measuring recalculation times on representative data is the most reliable way to evaluate performance.
How can I safely introduce dynamic arrays into an existing legacy workbook?
Start by identifying high-complexity CSE formulas that are hard to maintain or frequently modified. Prototype dynamic equivalents on a separate sheet, validate calculations, and then gradually replace the legacy formulas, documenting each change.
Keep a backup of the original workbook for rollback and clearly note the minimum Excel version required once dynamic arrays are introduced.