Excel: Create Unique Random Numbers

Key insights

• RANDARRAY
  Generates an array of random numbers with options for size, minimum, maximum and integer vs decimal.
  Use the last argument TRUE to return integers (for example, RANDARRAY(100,1,100,999,TRUE)).
• UNIQUE
  Removes duplicate values from an array so you can get only distinct numbers without manual filtering.
  Combine it right after RANDARRAY to drop any repeats automatically.
• SEQUENCE and INDEX
  Use SEQUENCE to request how many unique numbers you want and INDEX to return the first n items from the UNIQUE list.
  Example formula: =INDEX(UNIQUE(RANDARRAY(100,1,100,999,TRUE)),SEQUENCE(10)) — adjust the 100 (candidate pool) and 10 (count) as needed.
• Paste Special
  To keep results from changing, copy the spilled numbers and use Paste Special → Values to replace formulas with fixed numbers.
  This prevents recalculation when the sheet updates.
• Compatibility
  This method works on Excel with dynamic arrays (Excel 365, Excel 2021 and Excel for the web).
  Older Excel versions without dynamic arrays need helper columns or VBA to avoid repeats.
• Advantages
  The formula approach gives unique numbers without VBA, updates automatically, and is fast to set up.
  Control the candidate pool size (make it larger than the desired count) to reduce the chance you need to increase candidates for enough unique values.

Overview of the video

Aldo James published a concise YouTube tutorial showing how to build a random number generator in Excel with no repeats, and the video focuses on Excel features available in 2025. He demonstrates a single-formula approach that works across PC, Mac, and Excel on the web, assuming the user has a modern Excel build that supports dynamic arrays. The presentation targets everyday users who want unique random integers without writing macros or assembling many helper columns. Overall, the video aims to simplify a common spreadsheet task with accessible steps and clear examples.

Moreover, Aldo emphasizes practical use: generate a set of unique IDs, sample without replacement, or create randomized lists for testing and assignments. He highlights that the technique relies on built-in functions, so users can avoid VBA and reduce file-security concerns. As a result, viewers can reproduce the method quickly and adapt it to their own ranges and counts. The video keeps the focus on formula mechanics and on how to freeze results when needed.

How the formula works

At the core, the method combines Excel's dynamic array functions to produce unique values in one step. Aldo shows how RANDARRAY() creates many candidate numbers, UNIQUE() removes duplicates, and SEQUENCE() or INDEX() picks the first desired results from that filtered list. Because these functions spill arrays automatically, a single cell can generate the entire column of results without copying formulas down. This set-based thinking contrasts with older row-by-row approaches and reduces manual cleanup.

In practice, Aldo suggests generating a larger pool than you need and then selecting the top n unique items, which the audience sees as both simple and reliable. For example, you produce many random integers, remove duplicates, and then take the first n with SEQUENCE and INDEX. This pattern avoids iterative checks and uses Excel’s vectorized operations instead. Consequently, the formula stays compact while taking advantage of dynamic arrays.

Practical steps and freezing results

Aldo walks viewers through entering the formula, adjusting the minimum and maximum bounds, and ensuring the m parameter — the number of candidates — is large enough to yield n unique values. He also demonstrates how to convert the spilled results to fixed numbers by copying and using Paste Special → Values, so the random set no longer recalculates on every change. This step matters because functions like RANDARRAY() are volatile and update whenever the workbook recalculates. Therefore, freezing results preserves the sample and avoids unexpected changes in reports or tests.

Additionally, the video explains platform compatibility and offers tips for users on Mac and Excel for the web, noting that older Excel versions without dynamic arrays cannot use this single-formula approach. Aldo encourages viewers to test the formula on small ranges first, then scale up once they confirm behavior. He also reminds users how to switch between integer and decimal options depending on whether they set the integer flag in RANDARRAY. These practical touches make the tutorial useful for non-experts.

Tradeoffs and technical challenges

While the dynamic array approach is elegant, Aldo acknowledges tradeoffs that users must consider, such as choosing the right oversampling factor m. If m is too small, the pool may lack enough unique values and the returned list will be shorter than required, but if m is too large, you increase processing time and memory use. Thus, the viewer must balance generation speed against the risk of duplicates and consider the size of the numeric range when deciding m. This tradeoff becomes more pronounced with large n or tight min/max ranges.

Furthermore, the method has limitations: Excel’s random functions do not accept user-set seeds, so outputs are not reproducible across sessions unless you freeze them. Also, older Excel versions and some corporate environments do not support dynamic arrays, making the formula unusable there without alternate workflows. Aldo compares this method with legacy approaches — helper columns, RAND()+RANK(), or VBA — and explains that each alternative has its own cost in complexity, maintainability, or security. Therefore, choosing the right approach depends on version availability, performance needs, and whether reproducibility matters.

Takeaway for users

In sum, Aldo James’s video presents a clean, modern way to generate unique random numbers in Excel using functions that ship with current Excel builds. For most users on Excel 365 or Excel 2021 and later, this method reduces work, avoids macros, and integrates well with the spreadsheet workflow. He provides enough detail to try the technique immediately, while warning about volatility and performance considerations so users set appropriate expectations. Consequently, the tutorial serves both beginners and intermediate users looking for a practical, formula-driven solution.

Finally, Aldo’s guide encourages experimentation: start with modest pools, freeze results when you need stable samples, and weigh alternatives where dynamic arrays are unavailable. By balancing simplicity against performance and reproducibility, users can apply this approach in classrooms, data sampling, and daily spreadsheet tasks. The video thus fits well into the 2025 Excel landscape where dynamic arrays make set-based tasks easier and more reliable than before.

Keywords

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