Community
Free
For learning, teaching, academic research and non-profits.
- Every simulation feature
- Reports marked “Non-commercial use”
- No account needed
Nothing to install · free 30-day trial for work
Your Excel model gives one answer. Decide with the odds.
Monte Carlo simulation for the models you already have: the chance of hitting your budget, date or target, what drives it, and which decision works best.
- No add-in, no Excel needed
- Your workbook is never uploaded
- No sign-up to try
Runs in your browser on Windows, macOS and Linux; Chrome and Edge recommended. Nothing to install, no admin rights needed.
Why xellstorm
Your workbook stays on your computer
The engine runs inside your browser tab. Your workbooks, inputs and results never reach our server.
Parallel trials, partial recalculation
A Rust engine compiled to WebAssembly spreads the trials over your CPU cores, in most workbooks recalculates only the cells each trial changes, and can stop as soon as the results are stable.
Checked against Excel, on your file
When you open a workbook, its formulas are recalculated and compared with the values Excel saved in it. You see how many match, and any cell the engine cannot calculate is flagged, never guessed.
From workbook to decision in three steps
- 1
Open your workbook
Drop an .xlsx or .xlsm file. It is read and checked in your browser; formulas stay as you wrote them.
- 2
Mark what is uncertain
Your sheet opens as Excel shows it, with likely assumptions and results outlined. Pick an assumption to make it a range: it starts at 90% to 110% of your number, and can be linked to the Min / Likely / Max next to it. Pick a result to track it.
- 3
Run and share
See the odds of hitting your targets and what drives them. Export a results workbook, an HTML report or PowerPoint slides.
Start from a model like yours
Seven example workbooks open in the app with their inputs, outputs and targets already set, ready to run. No file of your own needed, no sign-up.
- Business case
What is the chance the investment has a negative NPV?
Base-case NPV USD 562k; negative in 29.6% of outcomes - Project cost and schedule
What are the odds of going over budget, or finishing late?
PERT estimates · risk events - Risk register
How much contingency does the register need?
Estimates linked to the register · worst-5% average - Inventory policy
Which reorder point balances cost against stockouts?
12 months of demand · scenarios - Tolerance stack-up
What share of assemblies will miss the clearance?
Simulated 3.2%, exact answer 3.29% - Equipment reliability
How likely is the machine to fail within a year?
Simulated 79.0%, exact answer 79.05% - Order quantity
How many units should we order when demand is uncertain?
Best in steps of 10: 1,090; exact optimum 1,086.1
Each one is worked through step by step in the learning center: NPV and IRR, project contingency, risk register, reorder point, tolerance stack-up, reliability and order quantity.
Where an exact answer exists, our test suite checks the simulation against it on every change to the engine.
Everything a risk model needs
A map of the model, not a maze of cells
xellstorm reads the formulas and draws the workbook as blocks of assumptions, calculations and results, named after the labels next to them. Pick inputs and outputs on the map or on the sheet as Excel shows it; likely ones are outlined for you.
From “what could happen” to “what should we do”
Find the order quantity, price or budget that maximizes the mean, a percentile or the chance of success, under risk limits such as “chance of a stockout at most 50%”. Goal seek and one- or two-way sweeps are there too.
Every candidate runs on the same random numbers, and the best ones are re-run with more trials, so you know whether the winner is really better or just lucky.
See why, trial by trial
Summary numbers hide thresholds and kinks. Plot any input or output against another, one dot per trial, with the trials in your target condition, such as cost over budget, in red. Spider charts show how each input moves the result, and worst-tail drivers show what pushes it into the bad tail.
Describe what is uncertain
48 distributions, including your own tables
PERT, triangular, lognormal, Weibull, discrete risk events and more, with SciPy’s parameters, plus the further families add-in models use; discrete, cumulative, general and Metalog tables and Trigen; truncate or shift any of them.
Estimates from your sheet, data or a workshop
Min / Likely / Max or Low / Base / High columns link to a PERT, so edits in Excel carry over. Fit to historical data by maximum likelihood, or to P10, P50 and P90 estimates.
Correlation matrices
Edit or paste a matrix, or estimate it from historical data. Matrices that cannot hold are repaired, and a scatter plot shows the correlation you got.
Time series and plans by year
Random walk, AR(1), geometric Brownian motion and mean reversion paths for prices, demand and rates; a row of yearly results reads as a fan chart.
Find what drives the result
Drivers for every output
Tornado, spider and worst-tail drivers, rank correlation and contribution to variance.
The bad tail, not just the average
The average of the worst 5% (CVaR), percentiles, and how often two targets are missed together.
Fair scenario comparisons
Scenarios run on the same random numbers, so the difference you see comes from the decision, not from different random draws.
Stops when the answer is stable
Latin Hypercube sampling, and a stopping rule that runs until means and percentiles settle within your tolerance, instead of a guessed number of trials.
Decide and share
Reports anyone can open
A results workbook with every trial, a single-file HTML report that prints to PDF, and PowerPoint slides with editable charts, including the plots you pick. Colleagues and clients read them in Excel, PowerPoint or a browser: no license, no viewer.
No lock-in
Your workbook stays a plain Excel file: xellstorm adds no functions to it, so there is nothing to strip out if you stop. The setup lives in a small project file next to it.
An audit trail in every report
Each report records the engine version, trials, seed and sampling method; the results workbook and HTML report also record the workbook’s SHA-256 fingerprint.
Runs you can repeat
The same model and seed give the same numbers, whatever the number of CPU cores. Save a project and rerun it later.
Moving from an Excel add-in?
xellstorm reads 164 distribution functions of @RISK, ModelRisk and Analytic Solver, their percentile forms, and NORM.INV(RAND()) style formulas, and turns them into inputs whose parameters stay linked to your cells. Cells marked with RiskOutput, VoseOutput or PsiOutput are offered as outputs, with the names they carry; correlation matrices become rank correlations, and RiskSimtable lists become scenarios. Open the workbook you already have: before you run, it lists what it converted and every formula it left, with the reason, and the compatibility check names any add-in function left to remove in Excel.
Try it on your existing workbook See every supported function
- Excel
- Not needed. Runs in your browser on Windows, macOS and Linux.
- Installation
- None. No add-in to package, no admin rights.
- Engine
- Its own calculation engine, spread over your CPU cores; it does not drive Excel.
- Optimization
- Included: optimize, goal seek and sweeps, on the same random numbers.
- Your files
- Stay plain Excel: no add-in functions written into them, nothing to remove later.
- Sharing
- Reports open in Excel, PowerPoint or a browser; readers need no license.
- Price
- $588 per user per year, or $59 month to month, with every feature included; a 30-day free trial, no card needed.
Privacy you can check
Calculated in your browser
The calculation engine runs inside your browser tab, so workbooks, inputs and results never reach our server.
Stored in your browser
Recent workbooks, settings and run history are kept in your browser’s own storage, on your computer.
Reports made in your browser
Results workbooks, HTML reports and slides are built in the tab and saved as files on your computer.
Locked down by the browser
The app loads nothing from other servers, and its Content Security Policy lets it connect only to its own address.
Check it yourself: open your browser’s developer tools, run a model and watch the Network tab. No request carries your workbook. IT teams can read the app’s Content Security Policy in its response headers.
Simple pricing, per user
Free for non-commercial use. For work, try every feature free for 30 days, no card needed.
Pro
$49 / month
billed annually ($588), or $59 month to month
For one professional, at work.
- Everything in Community
- Commercial use
- Reports licensed to your company
- 30-day free trial for new users, no card needed
Team
from $44 / user / month
billed annually: $49 for 2 to 9 users, $44 from 10; or $59 month to month
For teams of 2 to 25 users, on one subscription.
- Everything in Pro
- Add and reassign seats yourself
- One subscription and invoice
Enterprise
From $10,000
per year, billed annually
For large organizations.
- On-premises or air-gapped deployment, on request
- Invoice and purchase-order billing
- Custom terms, DPA and security review
Seats are only for the people who build and run models. Everyone else reads the reports in Excel, PowerPoint or a browser, with no license.
Prices in USD, or the same amounts in EUR, excluding VAT and sales tax. Orders are processed by Freemius, our reseller and merchant of record, who issues invoices and handles taxes. Card or PayPal; 30-day money-back guarantee. Can only pay against a purchase order? We invoice annual prepayments from $3,000.
Questions
New to Monte Carlo simulation. Where do I start?
Open an example model, or your own workbook. Pick the assumptions you are unsure of: each starts as a range from 90% to 110% of your number, which you can widen or link to the Min / Likely / Max columns next to it. Pick the results you care about, set a target such as “total cost above 2,900”, and run. You get the chance of hitting it and the assumptions that matter most.
Or read Monte Carlo simulation in Excel for the method, with formulas you can try in Excel itself.
How many trials do I need?
Start with the default of 5,000, or let xellstorm decide: it can keep running until the mean and percentiles settle within your tolerance. Latin Hypercube sampling, the default, usually needs fewer trials than plain random sampling.
Is my workbook uploaded anywhere?
No. The workbook is read and calculated inside your browser tab and stored only in your browser. Workbooks, inputs and results never reach our server.
Which Excel features work?
The formulas common in financial, engineering and project models, including NPV, XNPV, IRR, XIRR, PMT, EDATE, EOMONTH, WORKDAY, NETWORKDAYS, SUMIFS, INDEX, MATCH, XLOOKUP and OFFSET. Every workbook gets a compatibility check before you run it, counting any cell the engine cannot calculate.
Not supported: links to other workbooks, VBA macros and user-defined functions, and a few newer functions such as FORECAST.ETS; cells that depend on them are flagged, never guessed. Data tables keep their saved values during a run.
Which files can I open?
.xlsx and .xlsm workbooks. Save .xls and .xlsb files as .xlsx first, and remove the password from workbooks that need one to open.
Will I get the same results as my add-in?
The same model, not the same random numbers. Converted distributions keep their parameters, correlations keep their rank correlations, and anything that is not converted is listed with the reason. Once everything converts and the compatibility check is clean, means and percentiles differ from your add-in’s by sampling noise, which shrinks as you run more trials; with correlated inputs, each tool pairs the draws its own way, so the tails can differ a little more. What converts, function by function.
How do I get this past IT?
There is nothing to install and no admin rights are needed, but your IT team may still want to check. The app loads nothing from other servers, its Content Security Policy lets it connect only to its own address, and the browser’s Network tab shows that no request carries a workbook. For use inside your own network, Enterprise offers on-premises deployment on request.
Does it work offline?
Not yet: keep a connection while you use the app. It only loads the app’s own files; your workbook is still calculated on your computer and never uploaded.
What counts as non-commercial use?
Personal learning, hobby projects and coursework; teaching and coursework at accredited schools and universities; academic research intended for publication; and the own operations of registered charities and non-profits.
Work for a business, a government body or a client is commercial use, including freelance and consulting work, research funded by or done for a company, and evaluation beyond the trial period.
What happens when the trial ends?
Nothing is charged: the trial needs no card. Your workbooks, setups and runs stay in your browser, and xellstorm carries on as Community, free for non-commercial use, with reports marked “Non-commercial use”. To keep using xellstorm for work, choose Pro or Team.
How do seats work?
A seat is one named user who builds and runs models; people who only read the reports need none. Team comes in packs of 2, 3, 5, 10, 15, 20 or 25 seats, at $44 per seat per month from 10 seats, billed annually.
Can we pay by invoice or purchase order?
Pro and Team are paid by card or PayPal, and every order comes with an invoice that carries your VAT number. If your organization can only pay against a purchase order, contact us: we accept annual prepayment from $3,000, and Enterprise is always invoiced.
Government agencies and universities?
Government bodies use the commercial plans and can pay with a purchase card; orders that need an invoice or purchase order start at $10,000 a year, as Enterprise. Teaching and published academic research are free on Community; university administration uses the commercial plans.
Can I cancel?
Any time. Your plan runs until the end of the period you paid for, and every first payment has a 30-day money-back guarantee.
Is there a desktop app or an Excel add-in?
No: it runs in the browser, so there is nothing to install.
Put odds on your next estimate
Free for non-commercial use, and free for 30 days at work. Start with an example, or open your own workbook: it is calculated in your browser and never uploaded.