How-to

Task-oriented steps for running a Monte Carlo simulation on a spreadsheet, from upload through reading results. These assume you already know the core concepts.

How to upload a workbook

1. Go to App → Spreadsheet Sim.
2. Click Upload .xlsx in the header.
3. Select an .xlsx file from your computer.

The workbook opens in the IronCalc viewer in the main panel. Sheet tabs appear at the bottom — you can switch between sheets.

note

IronCalc is read-only. You cannot edit cell formulas in the browser. To change the model, edit the file locally and re-upload.

How to navigate the layout

The screen is divided into two areas:

Sidebar (left):

• Assumptions list with a + Add button
• Outputs list with a + Add button
• Test runs input (how many times to simulate your spreadsheet — more runs = more reliable results)
• Save Config / Load Config buttons

Spreadsheet (right):

• Full IronCalc workbook view
• Assumption cells are highlighted in blue
• Output cells are highlighted in green

How to save and load configs

A config stores your list of assumptions and outputs for a given workbook.

To save a config:

1. Click Save Config in the sidebar.
2. Enter a descriptive name for the config.
3. Click Save — the config is saved to your account.

To load a config:

1. Click Load Config in the sidebar. A list of previously saved configs appears.
2. Select the config you want to restore — assumptions and outputs reload automatically.

tip

Use descriptive names (for example, "Q4 forecast — base case") so you can find configs across sessions and distinguish between scenario variants.

Configs are saved to your account and persist across sessions.

How to add an assumption

An assumption is any cell in your model where the value isn't certain. Instead of typing a single number, you mark it as a range — and the simulation will try many values within that range to show you the spread of results. Each simulation iteration overwrites those cells with freshly sampled values before the workbook recalculates.

note

Assumptions are always single-cell references. You cannot attach a distribution to a range or a named region — target one cell per assumption.

1. In the sidebar, click + Add under Assumptions. The Add Assumption panel opens on the right.
2. Fill in the fields:
   • Name — a label for this assumption (for example, Unit cost).
   • Sheet — select the sheet that contains the target cell.
   • Cell — enter the cell reference (for example, B4).
   • Distribution — choose a distribution from the dropdown.
   • Parameters — enter the parameter values for the chosen distribution.
3. Click Add. The assumption appears in the sidebar list and the target cell is highlighted in blue in the spreadsheet view.

tip

Match the distribution to what you actually know. If you have historical data, normal or lognormal usually fit well. If you're working from expert judgment alone, triangular is a good choice — it requires only a pessimistic, most-likely, and optimistic estimate.

For the full list of assumption fields and supported distributions, see Reference.

How to edit or remove an assumption

1. To edit an assumption, click the edit icon next to it in the sidebar. The panel reopens with the current values pre-filled.
2. Change any fields, then click Save. The cell highlight and sidebar entry update immediately.
3. To remove an assumption, click the remove icon. The assumption is deleted and the blue highlight is removed from the spreadsheet.

How to run a simulation

The overall workflow is configure assumptions → set rep count → run → view results. A simulation run executes the Monte Carlo model you configured: for each replication, the engine samples new values for every assumption, overwrites the corresponding cells in the workbook, recalculates, and records your output cells. After all replications finish, you view the distribution of results for each output.

Before you run, make sure you have:

• At least one assumption configured
• At least one output configured
• A rep count set in the Reps field (default: 1,000)

note

Simulation runs are asynchronous. The server accepts your request immediately, processes it in the background, and the results panel updates automatically when the run completes.

1. Set the rep count in the sidebar (1–10,000). The Reps field defaults to 1,000; enter any value within the allowed range.
2. Click Run in the header. The server uploads your workbook and config, creates a simulation job, and starts processing.

After you click Run, the Progress indicator appears in the results panel showing the current status.

warning

The maximum rep count is 10,000 per run and is enforced server-side. If you submit a value above the limit, the server rejects the run with an error.

Progress indicator

A progress bar appears while the simulation runs. Pocketstats polls the job status every second, updating the bar as reps complete. Large rep counts (5,000–10,000) may take several seconds to finish, so the progress bar lets you monitor the run without leaving the page.

tip

Use 1,000 reps for quick exploration. Use 5,000–10,000 for stable statistics when presenting results.

What happens server-side

During each replication, the server:

1. Loads the workbook into the simulation engine.
2. Overwrites each assumption cell with a freshly sampled value.
3. Recalculates the workbook.
4. Records the values of each output cell.

After all replications complete, the server compiles the results and sends them back to the browser for visualization.

How to cancel a run

1. While the simulation is running, click Cancel in the progress bar panel.
2. The server stops the job and discards partial results. No data is saved.

Cancelling is useful when you spot an error in your assumptions, need to adjust the workbook, or started with too many reps by mistake.

How to troubleshoot a run

Problem	Likely cause	What to try
Run button is disabled	No assumptions or no outputs configured	Add at least one assumption and one output before running
Run fails immediately	Server rejected the rep count	Make sure the rep count is between 1 and 10,000
Run starts but never finishes	Workbook contains circular references or very slow formulas	Simplify the workbook model; reduce rep count to 100 for testing
Results show no data	Output cells are empty or contain errors in the workbook	Check that the output cells return valid values without the simulation
Server returns a timeout	Workbook is too large or has complex calculations	Reduce the workbook size or remove unnecessary sheets

For the full list of simulation parameters and their defaults, see Reference.

How to read results

After a simulation run completes, the results panel appears below the spreadsheet, showing the distribution of values for each output cell. For each output, you see:

• Histogram — distribution of sampled values across all iterations
• Summary statistics — mean, median, standard deviation, min, max, and selected percentiles (P5, P25, P75, P95)
• Pr(x < X) line — a draggable vertical line on the histogram showing the probability that the output is less than a given value

How to read the Pr(x < X) line

1. Locate the histogram for the output you want to analyze.
2. Drag the vertical line left or right to your threshold value. As you drag, the label updates in real time.
3. Read the probability from the label: Pr(output < X) = Y%. For example, "there is a 30% chance profit is below $0."
4. Adjust the threshold by dragging further until the probability matches your risk tolerance or decision criterion.

tip

The Pr(x < X) line is fully interactive — you can drag it continuously to explore any threshold. Use it to answer "what-if" questions like "What is the chance revenue stays above $1M?"

How to save results

warning

Results are not automatically saved between sessions. To preserve them, note the key statistics or take a screenshot before closing the tab.

For output types and the full percentile table, see Reference.