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Guesstimate
What it is
Guesstimate is a web spreadsheet for modeling and forecasting quantities that are uncertain. Cells can contain confidence intervals or datasets, and dependent calculations carry that uncertainty forward. Stochastic expressions use Monte Carlo sampling with 5,000 samples per expression. Analysis Mode displays sensitivity plots to show which inputs influence a model output. Calculators turn selected inputs into a form and show chosen outputs computed from the model. Financial functions include NPV, ROI, and WACC through Finance.js. The free plan allows unlimited public models. Anyone can view a public model, but only its owner can save edits; local changes made while viewing another person’s model are not saved. Paid plans provide private individual and organizational models, with organizational models visible within the owning organization. Guesstimate’s application code is open source, and its repository includes local-run instructions. QURI, a nonprofit research organization, says it maintains the product. The maker says small companies most commonly use it for cost estimation and forecasting.
For and against
For
- Uncertainty-aware spreadsheet calculations
- 5,000 Monte Carlo samples per expression
- Sensitivity plots in Analysis Mode
- Unlimited free public models
- NPV, ROI, and WACC functions
Against
- Private models require paid plans
- Only owners can save public-model edits
- Unsaved local changes are lost
- Web platform only
The verdict
Guesstimate focuses spreadsheet modeling on uncertainty, simulation, and sensitivity analysis. Its free public sharing is broad, while private work and saved collaborative editing require attention to plan rules.
Who it suits
Guesstimate suits small companies modeling costs and forecasts with uncertain inputs. It also fits users who want public sharing, sensitivity analysis, financial formulas, or open-source code.
On the record
- Analysis
- Analysis Mode uses sensitivity plots to help show how model inputs affect outputs.docs.getguesstimate.com · read 8 Oct 2026
- Calculators
- Calculators let users present selected model inputs and outputs as a form for repeated use.docs.getguesstimate.com · read 8 Oct 2026
- Financial functions
- Available financial functions include CAGR, NPV, ROI, and WACC.docs.getguesstimate.com · read 8 Oct 2026
- Free use
- The site says users can create unlimited public models for free.getguesstimate.com · read 8 Oct 2026
- Intended users
- The maker says Guesstimate is most commonly used for cost estimation and forecasting within small companies.docs.getguesstimate.com · read 4 Oct 2026
- Monte Carlo
- Guesstimate uses Monte Carlo techniques and performs 5,000 samples per stochastic expression.docs.getguesstimate.com · read 8 Oct 2026
- Notable limit
- Changes made while viewing someone else’s model can be edited locally but are not saved.docs.getguesstimate.com · read 8 Oct 2026
- Open source
- The FAQ says Guesstimate is open source and links to its GitHub repository.docs.getguesstimate.com · read 8 Oct 2026
- Organization status
- QURI describes itself as a nonprofit research organization and says it maintains Guesstimate.quantifieduncertainty.org · read 4 Oct 2026
- Paid model access
- Private individual and organizational models are available on paid plans.docs.getguesstimate.com · read 8 Oct 2026
- Private models
- Private individual and organizational models are available on paid plans.docs.getguesstimate.com · read 4 Oct 2026
- Purpose
- Guesstimate is a spreadsheet tool for modeling and forecasting uncertain quantities.docs.getguesstimate.com · read 8 Oct 2026
- Sharing limits
- Public models can be viewed by anyone, but only their owners can save edits; private organizational models are visible within the owning organization.docs.getguesstimate.com · read 4 Oct 2026
- Simulation
- Guesstimate uses Monte Carlo sampling and performs 5,000 samples per stochastic expression.docs.getguesstimate.com · read 4 Oct 2026
- Support
- Users can report bugs through GitHub or the site chat, and security-related bugs can be reported to [email protected].docs.getguesstimate.com · read 8 Oct 2026
- Uncertain inputs
- Users can enter confidence intervals or datasets directly into cells, and Guesstimate performs downstream calculations with them.docs.getguesstimate.com · read 8 Oct 2026
- Who it is for
- The FAQ says Guesstimate is most commonly used for cost estimation and forecasting within small companies.docs.getguesstimate.com · read 8 Oct 2026
