You can use JSON Logic to create complex calculated values based on the values of other fields.
Formio.createForm(document.getElementById('formio'), { components: [ { type: 'number', label: 'A', key: 'a', input: true, inputType: 'text', multiple: true, prefix: '+', validate: { min: 2, max: 100, step: 1 } }, { type: 'htmlelement', tag: 'p', className: 'text-center', content: '———— X ————' }, { type: 'number', label: 'B', key: 'b', input: true, inputType: 'text', multiple: true, prefix: '+', validate: { min: 2, max: 100, step: 1 } }, { type: 'textfield', label: 'Total', key: 'total', input: true, inputType: 'text', disabled: true, calculateValue: { "*": [ {"_sum": {var: 'data.a'}}, {"_sum": {var: 'data.b'}} ] } } ] });